(Maybe the answer is "because spells/abilities at higher levels aren't fully implemented yet", but that's not really true, they're just imperfect; they lack descriptions or are replaced with feats)
If you dont buy that answer i have another one for you ...
Bcs Swen said so.

Really tho, that is reason we get ...
It was planned since start that there will be level cap, nobody ever told us "why" exactly.
But i would presume that reasoning would be something about finish line, where developers can stop and go to next class ... it would take much, much and once more much more time if they were suppose to implement class with all its spells and level features right now. :-/