Because he isn't a winning head coach. So they're betting on unproven goods. Doesn't mean he won't be a good HC (I think he'll be fine) but they're talking about prospective national titles and being better prepared for the SEC, when he was the DC of a more talented than OU basically-SEC team that gave up 91 points in its last two playoff games.
Their anger at USC isn't JUST about Lincoln Riley, but it's an identity war thing and their inferiority complex in all things that have to do with them vs. perceived coastal elites. Oklahoma's a state that feels like it's often looked down upon by others and this is another example. (Remember how angry they got when KD left for GSW?) Every time Lincoln posts a beach pic or talks about living in paradise or USC's potential they take it as a personal affront. Not because he's saying anything about THEM but it's how it makes them feel.