Because they hated him when he was the QB. Because he 'sucked'. And so that's why.
There's no particular reason to be all proud of him either, that's just the other side of the coin. No need to hate or resent players that either sucked, or disappointed, or transferred or whatever. And you don't have to do it in the other direction, unless you had a particular fondness as a fan for a given player. A little bit, sure, but there's no reason to go overboard.
But as to why TR isn't fawned over, given what a hotshot phenom young coach he is? Because he sucked as a player, even though he really didn't, firstly. But quite obviously the main reason would be his very, very, almost inextricably close relationship with BK, who is truly hated by ND fans. I mean is this even a serious question?