I think we're overestimating just how hands on "Disney" is really involved here. They're fine collecting their cut of the Simpsons food, beverage, merchandise, and licensing fees, and while I'm sure they've explored their options, it doesn't feel like they're actively looking to add the IP to their own parks right now.
The Simpsons, as an institution, still has a fair amount of weight to throw around, even if it isn't the red hot cultural juggernaut it once was. It could be literally as simple as them being like, "Hey, we really want to finally do something for Halloween. Can we make that happen?" How do you think that conversation goes if Disney stonewalls them because they don't want to give Universal seasonal decorations because it might, uh... compete with their own Halloween offerings, I guess? (Hell, we don't even know the particulars of that approval process. Disney might not even get a real say!)