I'd think that the biggest things stopping any new major theme park resort from opening in the USA (Disney or otherwise) are:
- business
- regulations
- protestors
Business: You could argue that with air travel the way it is that it's pretty easy for someone to get to either of the two resorts, currently. Why build another?
Regulations: regulations are far worse today than they were 50-70 years ago. EPA, OSHA, local laws/permits/fees/etc. It's not a friendly environment to build much of anything, even if you're the government.
Protestors: People will protest anything. Tell them someone wants to build something nearby and they'll be out in droves. Tell them that something that is no longer viable will be torn down and they'll protest that, too.
70 years ago it was: I have an idea. I think I'll give it a shot.
Today it's: I have an idea. Better hire a team of lawyers to sort through the regulations and fend off the protestors... You know what? It's not worth it (both in the time/effort and money/capital involved).
Yeah - Universal is doing it in Orlando but they've had to fight these same fights. They also don't have a choice if they want to expand.