Because he's not very good besides being athletic. He is a PF yet there are 3 better PFs on the roster. Plus, he has no skills to play SF.
Depending on the situation and opponent of course. If we're facing off against Bynum or Howard i'd go with Lopez, but other than that Anderson can give us those 10 boards and still spread the floor. Lopez shouldn't worry though, he'll play a shiitload of minutes this season.
Starters:
Vasquez
Gordon
Davis
Anderson
Farouq Aminu
Bench:
Rivers
Lopez
Warrick
Henry
Miller