Other mistake: After Jack climbs up the ladder and through the floor he then runs right to the locked door, all this takes about ten seconds. Somehow in those few seconds Wang, Gracie, and the rest of the girls have all made it up the same ladder and coming around the corner when he gives him the signal to come to the door when there's no way they couldn't have all done it that quickly.
Other mistake: Early in the film, Jack says Lopan had "light coming out of his mouth" but Jack was already blinded and couldn't have seen that.
Answer: Because the battle between the Chang Sing and Wing Kong was between them and the truck.
Grumpy Scot