The thing is...they don't want to.
Way back when the thing was first happening, I asked a Disney source about it. I was told that Disney is well aware of the contract terms, and that they had no intention of trying to alter them.
Disney wanted the characters for many reasons other than use in theme parks. They are fine with Uni having the rides, and are quite content with just profiting off of the other uses of the IP.
Did you or anyone you know ever "try too hard" to date a guy or a girl, and had to be told "play it a little cooler, you're coming across as desperate?" Well, if Disney were planning to try to get the Marvel characters in their theme parks, do you think they'd go out and tell people "OH, HOW WE WANT MARVEL IN OUR THEME PARKS?" Or do you think they'd try to play it as cool as possible, so they don't come off as desperate, which puts them in a position of weakness? Or perhaps, a position of even weaker weakness?
Nah.
Both companies are currently fine with how things stand now. Honestly, it isn't worth that much expense and trouble just to be able to theme rides at one resort.
Four words: Oswald the Lucky Rabbit. How many people were REALLY clamoring for Oswald the Lucky Rabbit? And yet Disney got the rights to Oswald back from Uni. Sure, it was Walt Disney's (and Ub Iwerks') first popular character, but it's not like Disney absolutely had to have it. But Disney wanted it, and eventually worked out a deal where they got it. Maybe they'll do something awesome with Oswald, and I'm sure they tell stockholders they consider it a potentially valuable icon and franchise, worth the effort to get back. And I'm sure whatever that effort was will be miniscule compared to what it would take if they ever wanted Marvel in the theme parks. I'm just saying, if Disney wanted Marvel, they'd go for it, but they probably wouldn't telegraph that want before they were prepared to do something about it. If you don't think they have Imagineers blue-skying where Spiderman might fit in one of the parks, I've got some magic beans to sell you.
EDIT: I'd also point out that it's not uncommon for this "one resort" to generate more revenue in any given year than Disney's motion picture division. One of the reasons we're often such a surly bunch is because we feel that WDW is not living up to its full potential and if only people who cared as much as we do ran the parks then we'd see some serious changes for the better, I'll tell you what. Well, if someone like me were to be in charge of the parks, he'd assemble a small team to quietly explore any deal we could offer Uni that would get them to nix the contract. If someone like me were in charge of the parks (and, you know, thank God I'm not, because I don't know what the hell I'm doing), he'd even see if giving up rights to an Avatar experience, on top of a cash deal, might sweeten the pot. Or he might see if there's rights to a script or franchise not currently being adapted by Disney but controlled by Disney that Uni might want. And then someone like me might use the Marvel characters to make a giant expansion to seriously bolster the Hollywood Studios park, as well as see if there would be any way to leverage some of the more futuristic Marvel characters into Tomorrowland. Like, oh, I dunno, another retrofit of Stitch's Great Escape, which no one, even kids, seems to like all that much, and make it a Fantastic Four experience where Reed Richards has developed a portal into the future that Doctor Doom tries to steal. Or use Soarin' like technology for a Silver Surfer-themed ride.
Also, I asked if the contract with Uni was "forever" and was told it "might as well be". Take that for what it's worth.
And if Uni never chooses to cede their rights to the contract, that's how it will stay. But if Disney makes a deal that Uni decides is worth their while, then the contract can be broken. Uni's in the position of strength for now, but who knows if they'll ever want something Disney already has, and be willing to make a deal?