Couple reasons I think of.
1 is you lose your bishop to the white pawn next. Then the King has more mobility to get away from your queen/rook
And, you still haven't broken the stalemate of the white rook keeping your pawn from promotion. White just never moves the rook and black can't move his rook and that leaves a knight and two pawns against a king and two pawns... With a white rook covering 7, while the black rook just stares on. The bishop sacrifice at that moment gives black a checkmate in 3 (you're right not 2).
How do you see it playing out if black takes the pawn?
Imagine telling people the benevolence of the rich will lift everyone, in a system built to reward NOT BEING BENEVOLENT. With a straight face.