Lots of reasons. WRs, Josh and Brady all part of it.
Bills use its passing games as an extension of its running game. Basically a lot of short passes looking for yac. They want these long time consuming drives and it’s hard to do over and over again.
Everything is getting predictable. Brady is trying to disguise formations to get guys the same opportunities. It’s like every guy is only good for a couple things, Look how many times they throw bubble screeens to Shakir. Different formations, same play. Coleman’s jump balls, Kincaids crossing routes. There is very little versatility in each individual and that’s by design.
They rarely try to go to the intermediate and deep levels in the passing until they are put in that 3rd and long situation. If you refuse to attempt that stuff on early downs it makes it easier to cover on later downs. So at this point it’s just a major weakness because it’s not what they want to do and it doesn’t look like it’s practiced much. Even in long situations they still have a lot of short passes where they are looking for yac instead of just trying to get it to the chains.
Right now the short passing game is used so often D has no respect for the rest of the field. It’s like the Bills O is playing in a box at this point.