None of us (as far as I know) work for the Patriots, and therefore we can't know what happens internally outside of what we observe on Sundays (and Mondays/Thursdays ... and this year, Tuesdays).
That said, the vibe I'm getting based on the murmurs and whispers from reporters is that Stidham is talented, has a live arm, shows up at times in games, but isn't consistently performing well from a mental/leadership/work-ethic perspective in meetings and on the practice field.
Not sure if that is the case, but it would explain why Cam has remained the starter despite Stidham's seemingly higher potential as a passer.