Why using the queen is not a good idea in this case? I am a new [and a late] comer to chess. I started practicing on chessacademy.com where I got this exercise:

r1bqk1nr/ppp2pp1/4p2p/8/1bPnN3/5N2/PP1B1PPP/R2QKB1R w KQkq - 1 13


According to the site, the best move is ♗b4 and then it asks the following:


  [Why not accomplish the same thing by Qa4+...Qxb4?]


I can't see why not? Any hints?
You are missing the knight fork on c2. After 1.Qa4+ Bd7 2.Qxb4 Nc2+ 3.Kd1 Nxb4 you would lose the queen...