The queen cannot checkmate an enemy king by herself. Instead, the king and queen must work together to finish the game.
At this point, there's no need for White to move the queen again until he's ready to checkmate the Black king. Instead, he can bring his king closer to the action while Black is reduced to moving his king back and forth with moves like 5. ... Kb8 6. Kc4 Kc8 7. Kc5, which brings us to the position diagrammed above. Black's only legal move is 7. ... Kb8.
