3rd Version removing acess to Pokemon is so weird.
In Platinum you straight up can't get Murkrow or Misdreavous, and it's baffling to why they made it that way considering 2 important trainers feature their evolutions.
I mean, it's... just the abiding game logic. No game in the series has ever had all of its native Pokemon available in it (with the obvious exception of single titles like Colosseum, XD, and L:A). Yellow had its own version exclusives, as did Emerald.
It's not pleasant but it's been a part of the formula since day one. Significant trainers using Pokemon has very little to do with whether they'll be available for the player to catch, Blue uses Arcanine in both Red and Green and numerous trainers use Pokemon only accessible via trade like Gengar and Alakazam.
Yellow's missing Pokemon were at least ones which had some relevance, as they're mostly Pokemon Ash would have been unlikely to catch in the anime: Ekans, Koffing, and Meowth are obviously Team Rocket's Pokemon, while Weedle is a Pokemon Ash notably failed to catch in an early episode (though ironically he eventually went on to catch one in the Johto arc). Jynx, Electabuzz, and Magmar don't have much anime significance, but form a loosely-connected trio and Electabuzz/Magmar are exclusive to Red and Blue respectively, adding an extra layer of challenge since you'll need to trade with both versions.
I do sometimes wonder how much game balance was taken into account when removing Pokemon in third versions, though. Most paired versions do exclusivity with Pokemon of the same type so you've basically the same team options in each game - two Grass-types, two Fire-types etc - but, as people have pointed out, Mareep's loss in Crystal makes obtaining an Electric-type take much longer - if you don't get lucky with the Odd Egg and get a Pichu or Elekid, you either have to wait until after the third badge to catch a Magnemite on Route 38, a Chinchou or a Voltorb in Olivine, or else grind phone calls to get a Thunderstone to evolve your Eevee.
Emerald and Platinum's spread of missing Pokemon are a bit more forgiving - there's Grass-types aplenty so Roselia isn't a terrible loss, Meditite comes too late to be a problem, and odds are you didn't catch a Surskit in RS anyway. Though I recall there is a relative scarcity of Dark-types in Platinum since both Stunky and Murkrow are absent - Spiritomb is unobtainable in solo play, Drapion takes ages to evolve, and Sneasel comes very late. That basically leaves you with Houndour, which also comes relatively far into the game, or Umbreon which takes a lot of effort to evolve.