The very real possibility that the reason Stidhman isn't starting is because he isn't good enough and the coaches believe that Stidham is a worse overall option for the team even if Cam is struggling.
The "it can't be any worse" thing is a fallacy. Yes there are plenty of QB's in this league that are on the bench that are dreadful and worse than Cam. Cam at least adds to the running game, puts the work in, has experience, and players respect him as a talent. It's a very real possibility Belichick knows Stidham has none of that and is just a liability and a waste of time who isn't panning out that he's keeping around for cheap death because he's been in the system a year.
To be blunt, I trust Bill Belichick's evaluation of players on his team and, if after a few weeks with barely any practice, Bill decided that Cam was the better option and is still going with him. I am going to say that he's probably right and our guess work isn't worth questioning it. Especially since Stidham has been nothing but a complete disaster everytime he has been given a chance.
I said this in another thread, but Stidham is the type of guy that should be chomping at the bit to get an opportunity and make the most of it like Brady in 2001. Instead he's been a complete wreck. He hasn't earned the right to get a week of 1st team reps.