a couple of things Bon Vivant. Here are a few reasons why you see jerking to pop instead of fucking to pop.

1. the girl is brand new the director/talent don't think she can get in the right position to receive the pop on camera.

2. its the uncomfortable position for the male talent and he can't pop in that position.

3. the girl is on her period and either has a sponge or a Nuva Ring in - those hurt the big dicks - no way to fuck to pop.

I am sure there are more, but that's all i got for now lol