Don't know why. Robinho is also playing fine and it's Cassano or Robinho. I guess due to the fact that Robinho had more chances to play with Pato/Zlatan, Allegri gives him the edge.
I hope Pato will be out for at least 2 matches so Milan will have to play without him and Ibra, in addition to Inzaghi's absence. They will probably face problems having attackers like Cassano and Robinho up front and no target man.