Why not Qb3?

1. d4 Nf6
2. c4 e6
3.Nc3 Bb4
4. Qc2...

4. Qc2 is the mainline, but why not Qc2

4. Qb3 is dubious since it allows following continuation: 4... c5 5. dxc5 Nc6 6. Nf3 Ne4. Notice that after Qc2 it is impossible for black to play Ne4, since queen covers e4.

Thanks, that helped a lot!