We do know from earlier comments by Belichick and others that the Pats feel that they hope rookies make their big leap between their first and second seasons. And by all third hand accounts, that didn't happen. The confounder is that the 2020 offseason activities were all cancelled by CoVid, and even training camp was a bit truncated. But none of the press reports had Stidham outplaying Cam in preseason; in fact, Stidham was demoted to 3rd string QB behind Brian Hoyer. Then Stidham was injured all last season.
To be fair to Stidham, he's not had much opportunity. He also didn't seize the opportunity in the few chances he got. So, it's understandable why Belichick decided it was time to move on, and why McDaniels was willing to give him an opportunity, given the roster situation for both teams.