I think there's a lot of reasons.
S'mon said:
It's grindy because the designers wanted monsters to hang around long enough to use their powers, and tactical situations enough time to develop in interesting ways. They wanted to get away from the 3e 3-round fight towards more of a 6-round fight as standard.
I think this works well for big set-piece battles, but can be a problem in conjunction with 4e's emphasis on having several fights in an adventuring day.
I definitely think that's part of it.
Though it is support for my "we need stuff to do when not in combat" position. If, out of your 30-some-odd powers, only a quarter of them were attacks or combat utilities (with the rest being abilities to use in social situations, or in exploration), that would be fewer things to do in combat.
Fewer things to do = fewer rounds are OK.
Fewer rounds = shorter combats.
Shorter combats = more stuff (and less grind)
More stuff = a faster pace for the game.
a faster pace = a game that can be played casually, or a game that can be "stacked" onto itself for longer sessions.
more casual play = a game that can serve a broader audience.
So you can see that limiting options in this instance provides a better experience for more people.
The powers sections of 4e makes me think no one there was familiar with [ame="http://www.amazon.com/Paradox-Choice-Why-More-Less/dp/0060005688"]Barry Shwartz[/ame].
So many different kinds of blue jeans, all doing the exact same thing.