In The wisest things ever said about chess by GM Soltis, he quotes this about tempi and gives the following example:
Tempi should not be counted but weighed.
[FEN "r2q1rk1/pp1b1nb1/n2p1pp1/3Pp2p/2N1P2B/2N2P2/PP2B1PP/R2Q1RK1 w - - 0 1"]
[Site "Istanbul"]
[Date "2000"]
[White "Gelfand"]
[Black "Shirov"]
1. a4 Qb8
2. Nb5 (2...Bxb5 3. axb5 Nc7) Rc8
3. Bf2 Bh6
4. Qb3 Nc5
5. Qa3 *
The point of the example is that white started with 2 extra tempi in a King's Indian structure, that allowed him to place pressure on the queenside.
My question is, why didn't black play 2...Bxb5 3. axb5 Nc7 to gain chances to win one of the b-pawns?