The rule is worded "ordinary effort"

At the MLB level, you don't expect the pitcher to come off the mount to make that catch, so the only real possibilities are the 1B and 2B. The fact that the player had to (or at least chose to) leave his feet made the difference. You can see that the umpire started to raise his hand to call the infield fly rule, but as soon as the player left his feet he stopped, then gave the safe signal.

The MLB rule book uses "ordinary effort" in several places, and the way it is interpreted is pretty uniform. When deciding whether or not to give a player an error, the "ordinary effort" standard is used. If an outfielder dives for a ball and does not complete the catch, typically no error is given because the dive goes beyond "ordinary effort". Same with this play, as soon as the player leaves his feet in order to try to make the play, it goes from "ordinary effort" to something more than that - and therefore no infield fly is the ruling.