Because it makes the types incredibly bland.
Back in the first few gens a type informed what sort of role a Pokémon would play. Grass-types played around with status conditions and slowly healing; Dragons were rare overwhelmingly powerful Pokémon; Rock-types were physical titans; Electric-types were speed demons; etc. To a degree the Fairy type even practised something similar in its early days -- it was an undoubtedly amazing type, but the possibility of it being broken was kept in check by it mostly being given to Pokémon with mediocre stats.
The type matchups and common attributes of these types reflected this - Grass-types were only useful offensively against a few Pokémon so of course you wouldn't be trying to fire off super-powered Grass moves as you're just using the Pokémon wrong. Dragon moves weren't useful against anything except itself which was a rarity, so Dragons had overwhelming power but were kept in check by their STAB not being good against anything and not being too powerful. Electric-types were rarely incredibly powerful outside of Legendaries and their type matchup wasn't overwhelming, so despite their high speed they weren't singlehandedly sweeping anything they came across.
Exceptions did exist to each of these types, of course -- but that's what made them so cool and interesting. Sceptile was the fast and powerful grass-type and was exclusively given access to a fairly powerful grass-type offensive move that didn't require charging for that purpose. Lucario was the Pokémon that, aside from a lot of other interesting things about it; was characterised as a Fighting-type that actually had a special attack stat and had Aura Sphere for that reason. Heracross and Scizor the bug-types that could actually fight back; Aggron and Metagross the Steel-types with offensive power; Dusclops the tanky Ghost-type; etc. Their being exceptions to the rule made them stand out because said rule made you grow accustomed to what role each type filled and it was surprising how well these Pokémon could go against that.
This was not a perfect system, by any means; some types like Bug could barely be said to have any role beyond an in-game "it's kind of good for the first few hours but gets rubbish quickly" thing, and others like the Ice-type had a completely contradictory role that led them to be incapable of much of anything; the infamous 'slow glacier' archetype that makes them so awful to use. But the general system of each type having an archetype being the main thing about them rather than which types they do and don't have a 2X damage multiplier against was far more interesting and informed how you thought of different Pokémon. It's also resulted in balance issues -- Dragons were never meant to have powerful STAB moves, but then Gen 4 just gave physical Outrage to all of them and multiple Dragon-types now had to be banned and they had to patch in the issue by literally making a new type that was immune to them. The moment dark-types got given Knock Off they ran wild through the metagame, and when an offensive Ice-Type gets to exist all hell breaks loose.
As-is now, by just going with damage multipliers as what defines a type and asking to double down on this by literally wanting each type to just have clones of each other's moves, all you end up with is the current landscape where what the interesting types are is decided by which types they're good against, and that's that. As such it just means that Grass, Bug etc. end up having their identity as The Bad Ones, while Fairy, Dark and a few others have the identity of The Good Ones. It's just such a... boring approach to designing the main way Pokémon are differentiated, and won't fix any problems nor make anything more interesting just because Aurorus can now have a secondary special STAB.
