I've noticed that some Black/White pokemon evolve at very high levels - for example, Deino evolves to his 2nd evolved form at level 50, and only on level 64 you'll have a fully evolved pokemon. Why? I'm not asking only on Hydreigon or Volcarona, but for other pokemon like Miensho and Bisharp.
A lot of these Pokemon are strong. Really strong. If you look at their stats, a lot of these first-formed Pokemon are as strong as second-formed. Game Freak likely made them stronger at the expense of higher evolution levels to add a new gaming curve to the game.
This generation, everything is about bringing new things to the table. This tactic is probably going to get casual players to grind more levels and develop patience in the game.