>Flapple and Appletun evolution items can easily be found in the wild, making them easily obtainable pokemon that are strong for the early game but end up struggling a in the late game
>Driplin is only obtainable in the midgame and is weaker than it’s two counterparts making it a less reliable mon that exist to fill a similar role to magikarp in gen 1: a weak mons that reward the players that use it regardless by eventually becoming a powerhouse in the lategame (hydrapple)
All three of those mons to fill distinct role in the game that many types of players can enjoy. It’s the same reason as to why stone-based evolutions have their ups and downs compared to level based evolutions or how it’s easier to evolve g-slowpoke into g-slowbro(a average pokemon) than into g-slowking (a much stronger pokemon). Giving the two original dragon apple would simply ruin the game balance on the entire lines, making it more stale and boring to use in game and ruining its identity.