After Qb3 or Qb5, black has the move Rb4 - kicking the queen away from the defense of the knight. In that line, white is losing a full piece. There is slightly more compensation by instead playing Nxb6 Rxc4 Nxc4 because at least there the black king is also exposed.
If you're ever confused on why the engine played the move it did, just turn on the analysis afterwards. Make the move that you expected the engine to play and see why that loses worse than what was actually played - that way you can learn without having to ask an online forum for help :P