Because SFA3 didn't do all that well sales wise and of course Capcom with their infinite wisdom attributed that to "no one cares about the single player content".
It happens in other game series too. There are good features in less well received games and developers tend to overlook the game almost entirely when making the next one.
I can sort of understand why as they didn't have the benefit of social media and everyone having access to the net to properly gauge player response to features back then, but they damn well should know by now that world tour was one of the best aspects of that game. Why this is not a staple mode in sf games is still mind blowing.