I have a vague idea why, but are there any specific reasons?
Here is Junichi Masuda's explanation, direct from his (unfortunately inactive) blog:
In this Pokemon Black/White, only new Pokemon appears till the ending. Only new Pokemon appears! Reason why we come up with this conclusion is, we wanted every player to stands on a same line when they start the game. From Kids to adults. Also, every encounter of Pokemon would be fresh like player experienced in Pokemon Red and Green. The impression of the people outside of the office are… “It’s fresh to encounter new Pokemon every time” “Hard to find Pokemon’s type. Also their moves, too” “Feels like playing brand new game like Pokemon Red/Green” And so on… All impression were favorable. 3 August 2010
In this Pokemon Black/White, only new Pokemon appears till the ending. Only new Pokemon appears!
Reason why we come up with this conclusion is, we wanted every player to stands on a same line when they start the game. From Kids to adults. Also, every encounter of Pokemon would be fresh like player experienced in Pokemon Red and Green.
The impression of the people outside of the office are… “It’s fresh to encounter new Pokemon every time” “Hard to find Pokemon’s type. Also their moves, too” “Feels like playing brand new game like Pokemon Red/Green” And so on… All impression were favorable.
3 August 2010
Gen 5 was not the first time Masuda pulled a move like this; the Gen 3 regional dex is also very sparing with past-gen Pokemon. He hasn't done it again, largely because of public scrutiny, especially in Japan. Gen 3 in particular caused a dexit-lite where a lot of past-gen Pokemon were wholly unobtainable until FR/LG released.
