Is it?
I assume you’re referring to their reluctance to introduce new evolutions for old Pokémon with existing evolution methods, presumably in order to provide consistent explanations for why those Pokémon hadn’t been available before (i.e. if Eevee could be evolved with a Leaf Stone, Leafeon should have been obtainable in Red and Blue). The reason I find this so odd, though, is that there are lots of other changes which apparently aren’t ‘big issues’ at all – things like which moves a Pokémon learns as it levels, or Magnemite’s type, or the minor adjustments to their base stats received by many Pokémon in X and Y. Heck, you could even argue they’ve retconned an evolution method by what they did with Feebas and Milotic. I think the reason this particular aspect is so privileged is probably because obtaining Pokémon is a central objective of the game – they’re not so much interested in providing a reason why we couldn’t get Leafeon (or whatever); they’re interested in providing a reason why Professor Oak didn’t require us to. That’s just a guess, though.
Pokémaniacal 