jolie enfilade!

3. Nxd7

and either the queen takes knight and is lost, anyway, or the queen retreats to the only safe haven: c8. This will lead to white eventually having rooks on both the central files, and bishops that point towards blacks castle. The material gain for white in this case is +5, as opposed to +6 for 2. ... Qxb5.

Hence, 2. ... Bd7 is better in terms of material but so devastating in terms of position that it is better to accept the loss of the queen right away with Qxb5 and come away with a better game.