Because Instant Replay does not work and never has. There are too many loopholes. It doesn't take a Phi Beta Kappa in Physics to understand that if the left leg is closer to the sidelines than the right leg, and it gets down in bounds then without the push the right leg would be coming down in bounds too.
The loophole that has hung us twice now as you point out is that the Refs on the field have called it a catch without signaling a push out. In other words, they are saying the catch was made without the push out. A push out can not be challenged because it is a judgment call. But possession can be. If the WR gets both feet down it is a catch. If he doesn't, it is not.
So while the correct call (which is the whole stupid purpose of replay) would be to call both instances a reception, because of the stupid loophole it is not.
Replay does not work in its current form, and I doubt it ever will. All it does is disrupt the game.