Because he's terrible... Perk is a career backup player, but because the NBA is so weak in the middle these days, he's a starter who has numerous limitations in his game.
But I just wish he'd learn to keep the ball above his head or stop with his incessant need to take that one dribble on an offensive rebound... I have little hope that Perk will ever be more than a guy who makes layups or a right-handed jump hook as the whole arsenal of his post-up game...
Well, I wouldn't call him terrible, Andy. He's a bonafide defender down low for us. I think if he played in that Game 7, we could've overcome the Refs - errr

I mean the Lakers.
I miss the old days, too - Moses, Hakeem, Zo, Dikembe, Chief, Patrick, Admiral, Kareem.....I love Yao to life, but the poor man can't stay healthy.
Bogut and Kaman are solid offensive Centers, but they don't stop people.
I thought Perk was well on his way after that CHI series in 08-09, myself...he seemed to pick up a lot of confidence with that series.
I cannot really argue with you on the backup comment - Dwight Howard would backup those centers I named.