Slips hit the nail on the head when it comes to jumping. Anti-aring/trip guarding is match up specific due to different jump arcs and different hitboxes, compared to mk9. In fact, one could argue mk9's anti-air system broke away from traditional fighting games by having each character have universal jump in attacks and jump arcs.
Even in USF4, not every character can anti-air well in certain matchups. Divekicks can also skew the timing of anti-airs as well. Not saying anti-airs are great in MKX, they can be improved, but a lot players are so accustomed to mk9's style when most fighting games have different jump arcs/hitboxes for all characters.