Because leveling was hard to begin with, right? Everybody knows from the start what build they want to use and allocate points accordingly, also the talent trees were pointless since 1 combination of talents was the best and nobody used the rest.
Now they have baked the talent trees and the specialization into one, and gave you the option to choose a new spell/ability every 15 levels. How is removing a practically superfluous system and presenting you with real choices during leveling is dumbing down?