I'll use the Charmander line as an example. Why does evovlving a
Charmander into Charmeleon remove Charmander from your Pokédex,
same thing for Charmeleon to Charizard? Having the Pokédex work
like that kind of forces people (who care about it) to have a
living Pokédex.
This was done on purpose ^^ Pokeheroes is a collecting site. If the
pokedex was as simple as in the real games, there'd be no point to
having a pokedex at all because collecting the pokemon would be
extremely simple.
It's not really as hard as you make it out to be though? :) Just
remember to put on everstones and you're good.