Because in addition this being a Patriots board, he represents to many the definition of a phony. Again, he never won a championship without being either (a) the worst rated passer to win SB MVP during a postseason where threw 3 TDs, 7 INTs and won a game despite not scoring a TD, and (b) a season where he was benched for Brock Osweiler, finished with the worst passer rating of the Super Bowl era for a SB winning QB - worse than Dilfer, etc., and completely crapped the bed as always in the SB but got bailed out again.
Manning never won a championship by being gutsy, often had awards handed to him without earning them, and yet somehow receives the reputation as the guy who would succeed anywhere, despite being carried to titles and despite having tons of talent.
People resent that.
He's an all-time great...but there are other QBs who achieved more, and who earned their championships with numerous examples of clutch play down the stretch.