Universal is in competition with Disney.
They are seen as rivals. People like to pick sides when they see rivalries.
You have Coke drinkers vs. Pepsi drinkers, McDonald's vs. Burger King, sports team vs. sports team, etc.....
With the Disney parks having been around longer than the Universal ones, then naturally people are going to tend to be more loyal to the brand they already have experience and a history with.
This person nailed it, but its not rational behavior at all and I dont think its natural at all either. Natural behavior would be a constant evaluation and experiencing various things to determine if you still like one more than the other, what instead happens is blind loyalty. America is world famous for its partisan politics compared to other countries, I think it extends far beyond politics and is just a part of culture.
The other thing I noticed on this board is people like to bash anything thats not their favorite, be it universal, a different hotel and so on. Its really common among people who never even have gone to whichever place, which is amazing. If i had a dollar for every person that posted in a thread about x hotel instead talking about their favorite hotel and saying why x hotel is bad, only to say then they've never been there. I bet there are 5 times as many who don't even say that.