Disney's issue compared to Universal is the resort rooms. 30,000 to 9,000 -- at 75 percent capacity, that's 67,000 people to 20,000. Both parks would have tough time saying to any resort guests they can't go to a park on any given day, but Disney would have a larger number of people to deal with. I'm sure WDW would love to have a phased opening of just the monorail hotels to see how it goes, but that's a very bad look to only let the most expensive hotels in and cancel everyone else. Universal could open to resort guests only and have manageable crowds. For WDW it's much harder.
If UO opens first, my guess it's because they are in a better position to limit crowds than WDW is.