A lot of that is on the CBA:
https://www.bloggingtheboys.com/2015...t-tough-on-nfl
The NFL has limited teams to 11 padded practices for the first eleven weeks of the season. As a coaching staff, you are only allotted to have two padded practices in a one week period once. After that only one day per week can there be full-contact. In the final six weeks of the season a team is allowed to only conduct three padded practices. It just seems like a lot of time away from the actual game-type situations. Of course, the NFLPA fought for some of these provisions including limiting the amount and types of practices that can be conducted during the bye week. That's not to say that all players were in agreement about the changes but what's done is done at this point.