The creators wanted to make a change and make it harder to obtain good Pokemon. Previous pseudo-legendaries like Dragonite, Tyranitar and Salamence typically have very high evolution levels of level 50+ for the final stage. But in B/W they made it even more difficult.
For the Deino line it's level 50 to get Zweilous and level 64 to get Hydreigon. Some of the later Unova Pokemon like Mienshao and Volcarona also have high level evolutions.