Why would they do that? Usually you can start breeding after the second or third gym.
For the answer to that question you'd have to ask Game Freak. All I have is a couple of theories as to why they did things that way but that's what they are, theories. Sorry.
Official Cresselia of the B/W2 Boards; when will there be an official Cresselia event? Official Darkrai of PMD: Magnagate and the Infinite Labyrinth Board
Whatever the reason, its pretty dumb to not allow breeding until postgame. People can still breed in Black 1 and White 1 and just trade the pokemon over to Black 2 and White 2. It really makes no sense.
"you cant call it piracy if they wont sell it to you in the first place." -Fallenswords FCs, PSN and Gamertag in profile