For Normal-types with a secondary Type, there seems to be something about them that makes them have a lesser trait of say a Pokemon that's purely of is secondary Type:
Normal/Flying: Just doing all of them at once. They all are birds without an elemental affinity. Now why aren't they pure Flying? For some reason, at the time, GF had a rule where Flying couldn't be on its own. I guess because Flying wasn't strictly seen as a wind/air Type but more of a trait which could manipulate wind & air or had to do with birds (if that makes any sense). That's why for a long time it was only a secondary Type and the first pure Flying was a wind/air elemental. But back to Normal, since Flying couldn't be on its own and these Pokemon were elementaless that means Normal was really the only choice to be paired with them.
Jigglypuff family: Why was Clefairy made pure Fairy but not Jigglypuff? I would say has to do with how they move around. Clefairy are able to hop and float around via magical means, looking as if they're using an anti-magic force to make them lighter then air. Meanwhile, Jigglypuff moves around in mundane ways its body allows: clumsily walking/running, skipping, rolling, and inflating its elastic body with air to make it more buoyant. It's not really a Flying-type as it's not controlling the wind/air to make it fly, its just filling itself up. Jigglypuff is also more closely related to the sound mechanic than Clefairy which has been classified as Normal-type.
Girafarig: A case of duality between normality & the strange. Girafarig's front half looks perfectly normal, it's just a cartoon giraffe. But then you see it's back end and what happened? Its color scheme swaps and its growing a second head with a second brain of its opening, giving it more brain power than most creatures. While other Psychic-type on their own had the intelligence or power to be Psychic-type without complication, Girafarig's brain alone isn't enough so needs that second head to make it more than just Normal, but need to to have the Normal-type to do so.
Azurill: This is more of a question of why isn't it Water/Fairy like its evolution. I think it's the tail. Azurill's tail is bigger and heavier than it. When Azurill goes into the water it can't do any water acrobatics like its evolution because its tail is too buoyant limiting its movement. The tail itself is made of a elastic material it can bounce on like rubber, likely also waterproof. With it unable to bond with the water as deeply as its evolutions can it doesn't have the Water typing yet BUT still has that potential to be, thus it's only Normal-type to "fill the void" until it can be filled upon evolution.
Bibarel: Now Bibarel can manipulate the water... just not in the way most Water-types do. While many Water-types are just water elementals or almost lives entirely in water, Bibarel uses structures it builds to manipulate water flow. Most of these structures are made of wood & mud but it doesn't have control over the wood mud. It's just very knowledgeable about dam building and how to make a water structure both sturdy and aquadynamic, thus giving it unconventional control over water.
Mega Lopunny: Mega Lopuuny has a sleek fighter underneath all its fur... the problem is it has to shed that fur. Mega Lopunny's fur soften most strikes that would do a lot more damage other Fighting-types would be able to use without trouble, so it had to improvise. While it does try to shed off the fur on its legs so it can use its more developed legs to give strong kicks, it also has learned to use its segmented ears as nunchucks to deliver strong hits via centrifugal force.
Mega Audino: Going to say it has to do with sound being Normal-type. It got the Fairy-type in order to gain access to more healing & calming power (useful for a Pokemon that's a nurse) but its main way of diagnosing a patient is via using its stethoscope ears to listen to the patient's body sound. It retaining its Normal-type gives it the keen hearing sense to do this whereas if it was pure Fairy it's sense may be dampened by being more sensitive to the magic forces around it.
Deerling family: So, while Deerling and Sawsbuck has plant antlers, that's really the only Grass-type thing about them. Otherwise they're a normal animal. "The same can be said about Skiddo and Gogoat". Yes, yes it can, but they do have more of their body covered by their Grass-type trait while for the Deerling family its just their horns. Their body changing depending on the season could also have something to do with it, especially with Sawsbuck, maybe showing despite being the same being the two parts are still different enough they both go through their own transformation. Sawsbuck's plant antlers go through a typical tree seasonal cycle while its body grows & sheds fur, two different adaptations yet Sawsbuck displays both.
Meloetta: It's the sound thing again. Meloetta is a musical performer, while its voice has a soul soothing effect and dancing is full of vigor, no matter what form it takes still needs that singing connection. But to maybe dive a bit deeper, unlike other Psychic types its no using pure mind power to use its Psychic power but is doing so through its voice. Similarly, while it can deliver strong strikes it does not do so the usual way Fighting-types do via a trained martial arts but through another rigorous way to train & exercise the body, dance!
Diggersby: Pretty much the same excuse as Bibarel. It's not controlling the dirt it digs up but is using its digging and tunneling knowledge to manipulate the land to its whim.
Stufful family: And it shares a similar excuse with Mega Lopunny. Both are firmly packed with stuffing-like muscle it can break and squeeze hard, but at the same time when not tensing the stuffing its soft to the touch which is also how it absorbs physical damage. It's another duality, but this time between softness and hardness.
Oranguru: Now Oranguru could have been pure Psychic, however it's Normal-type because it chooses to hold itself back. Because what's probably the most normal thing in the Pokemon world than a human being? Oranguru likes being a trainer instead of the battler, and even when out on the battlefield has learned a special move it can use to command another Pokemon.
Drampa: Dragons usually are to extremes: fierce looking or cuddly (or are both). However Drampa doesn't really fit either, instead of being an extreme it more chooses to be in the middle. It's a gentle giant who is friends to all children, but also a berserking feathered serpent burning down homes of bullies to the children it likes. It's your dear old Drampa!
Alolan Rattata family & Galarian Zigzagoon family: Dark is a Type added onto what were originally Normal-types. For A-Rat they had to get meaner and more aggressive in order to combat their sworn Alolan rival: the Yungoose family. For G-Zig it's both them needing to become more dominating to compete for survival in the Wild Area but they also keep their Normal-type due to the sound connection as they're also loud. Technically their behavior and goals are the same, it's just that the regional variants are more extreme versions of this behavior because their survival depended on the species hardening up.
Litleo family: This I think it has to do with their looks. You look at other Fire-types and they either have an open flame on their body or their mainly brightly colored red, orange, and/or yellow. But looking at Litleo and its evolutions you'll notice it sort of mimics this but never goes full way in one direction. It's mane/crest/hair looks like its on fire but it's not, and while their hair is brightly colored the rest of its body is a dull black and brown. They have the element of fire, but rather then actively display it only front the element, probably so they can save it for only attacking... And there's once again that sound connection...
Helioptile family: Now, while they can generate electricity, they can't do so on their own. They need to absorb light & energy, not necessarily heat, in order to power their electric cells. Infact, despite living in a desert area, their Ability show their weak to heat. Any energy they get from eating is probably needed to keep it moving so it doesn't suffer from heat exhaustion. However, while that helped with the heat, didn't do much for the light, so probably adapted light absorbing organs as another way to keep it cool (and it turns that extra energy into electricity to not only use that energy but probably uses the excess electricity to increase its speed and reflexes).
Indeedee: Probably the same reason Mega Audino isn't full Fairy. Indeedee developed the Psychic-type to help them better serve their owners by reading their minds and emotions but uses Normal-type to dampen its power as it wouldn't be very hospitable of it to read your deeper thoughts.