He went from savior to traitor in the speed of a text. 11th hour betrayal like that lasts for a while.
If he sucked, we wouldn't care, but he's a pretty good QB, so it makes it more painful, thus, more hate.
It's just the way it is.
Oh, and maybe you haven't noticed, but Nebraska football fans take Nebraska football very seriously, which is probably why an unlikely program in the middle of nowhere with zero recruiting advantage is considered one of the elite programs in college football.