MCA/Universal had contract rights to Marvel for the whole USA and the Mississippi separated the "east coast" and "west coast" rights. If one park did not use the rights they would default back to the parent company, Marvel, but the rest of the contract would still be upheld. USH had several Marvel things at the park including a show (OF COURSE) and a
restaurant. In the end, they gave up the license in 2007 as they weren't using it which allows Disney now to build their Avengers Campus, just as the contract doesn't allow them to advertise it as MARVEL.
As for why Spider-man never came to USH, well I think
@Paulio provided probably the best reason why. I'm not sure exactly why it didn't as I was in college/fraternity/too cool for theme parks phase.