Maybe because he has proven to be important, something Giovinco has yet to do. You can't just look at stats, specially when it comes to Gio. Do you realise he has maybe scored one or two important goals? Goals that actually counted in the outcome of the game. He only scores when we've already won a game.
Here are his goals:
Udinese (4-1), 2 goals, 3rd and 4th goal.
Roma (4-1), 1 goal, 3rd goal in the 90th min.
Nordsj (4-0), 1 goal, 3rd goal.
Pescara (6-1), 1 goal, 4th goal.
Chelsea (3-0), 1 goal, 3rd goal in the 90th min.
Torino (3-0), 1 goal, 2nd goal.
Sampdoria (1-2) 1 pen goal.
I can count one important goal against Cagliari in the coppa and two assists against Bologna and Chievo.