I see this question pop up frequently and I too am wondering. I have an idea and invite anyone else with insight to jump right in.
We're on tape and there are stats. Early on with a new QB and a fair number of new offensive players opposing DCs may have used a more balanced approach that did not assume the run was going to stuffed by the front 4 or that the long ball wasn't a threat.
The pop pass requires the middle of the field to be open because the linebackers are cheated up towards the LOS and the safeties have either also crowded the line, where drawn into coverage or are playing deep.
More specifically the pop pass is an RPO. Last year Sam Rogers was the typical recipient of this pass with any of Peoples, Cunningham or T-Mac staying back to block. This year T-Mac was the recipient of one that went for a nice gain. After that we more or less saw Sean Savoy as the recipient which was an interesting wrinkle and I would assume would keep defenses guessing but we really have not seen it used with our running backs recently.
Why? I think this comes down to knowing Jackson is not a running threat. Jackson has a read on the RPO and if I remember correctly the read is on a linebacker. If the linebacker steps up to take Jackson, Jackson pops the pass into the spot he vacated. If not Jackson pulls it down and runs. Here in lies the problem. The linebackers are sitting still on this play and not vacating the middle. Once again this comes to down to blocking both on the line and by the skilled position players. The loss of Nijman has been huge. I know there was lots of criticism of his less than stellar play but we went from having a less than stellar LT to a plainly bad LT. If your lineman are getting beaten on every play there's no need to get your linebackers involved because your DE's and DT's are living in the opponents backfield and that disrupts both running and passing plays.
Not having the threat of the RPO pop pass really hampers this offense because the RPO is part of what is used to keep defenses on their toes. When a defense completely ignores the RPO it creates ripples that affect the rest of the offensive game plan.
It would also seem that Bush is not really a true threat either because most defenses play him like he will never pass and so do we. In this case Bush has to make at least one person miss and that has not happened.