after 1... Bxf4

White can play either 2.Ng2, 2.Kh1 or 2.Kg1

which in all cases stop the threat of 2... b1=Q because now 3. Rxb1 is available

The move Bb4 has two functions: clearing the way for a possible discovered check in the second rank, and shielding the promotion square from white rook. On the other hand, the move Bxf4 only clears the way for the discovered check, but doesn't protect b1, so promotion is no longer possible.