We've rehashed this one. Drew was a nearly 10-year veteran coming off a 5-11 season and had played poorly the first two games of the season and they were 0-2. Belichick, having also been a guy who knew how defend him before he became the coach, by then knew what he had and Drew, as much as I enjoyed him as a player, was who he was.
Mac was coming off a rookie season where he went 10-7, outperformed his rookie peers, and took the team to the playoffs. He was barely into his second season in a completely different offense when he got hurt. It's not - and still isn't - a fair comparison. If Mac went 5-12 last year and was still struggling three games into this year, maybe they would have stayed with Zappe. But that's not what happened.