why doesn't white take the pawn

I don't know how to post the board, so I'll just post the link to the game.  On move 11, why doesn't white play Qxb7?  It seemed like a good move for my opponent at the time, but even when I do the computer analysis, the move is not suggested.  Doesn't Qxb7 win either the rook or the knight?

http://www.chess.com/livechess/game?id=570584354

interesting, i can't say. that said, the piece can be saved by Kd7, but it does look rather dangerous with O-O-O coming, so black probably has some random trick to win something. :P

Off the top of my head Black can play Kd7 to avoid losing a piece at which point whther or not White casltes on the Queenside Black has play down the open file. White can retreat the Queen on the next but even after something like 12. Qa6 Rb8 13. Qd3 Rxb2 White seems to have some problems. I'm not doing this with an engine so I might not be suggesting the best moves, but I think Black has this *kind* of attack if White grabs the pawn. (If White doesn't retreat the Queen right away then Rb8 comes with tempo... maybe White has a useful move to make in the meantime 12. e3 or 12. e4... certainly 12. o-o-o must be risky after taking the pawn and opening the file.)

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White can safely take the pawn.  At best guess,  black looks to be forming up a nasty attack and white maybe did not want that open file on his castled king, or feared something with the knight coming down, or the like.  There is nothing (black is forming a nasty attack but its not there yet) but a moment of uncertainty. 

My computer took the pawn, black played Kd7 as others said, and then white castled 

Nd4 looked good at a couple of different times for black.

Thanks for all of your comments.

Update from home with an engine. After thinking for a very short amount of time Houdini thinks Black is better if White takes the pawn, not winning, but better... however Houdini also thinks Black is nearly winning if White doesn't take the pawn. I don't think you need a computer line as your question was already answered... just fyi the engine thinks your position was already extremely promising.