I think you are mistaken on how much money they are making off of it. You are just assuming they are. And if they owned DC, they could transform those attractions. Then it becomes an opinion that is divided on whether the DC brand can be as lucrative as the Marvel Brand. Universal is not making what you think off Marvel. And the parks do not draw as much due to Marvel. Just look at the huge attendance increase due to Harry Potter. They weren't even in the same galaxy as Disney till Harry Potter, and even now they still have a long ways to go. Moreover, as others have pointed out, Marvel is actually not as big of a draw in park entertainment as many other IPs are. You just can't get as immersed in a Marvel area as you can things like Harry Potter, Star Wars, and so many others. And that is because the area of "Marvel" isn't that much different than anything else. It's a city in most cases. Regardless, A good ride can stand on it's own. You wouldn't put "Betty Boop" to replace the Hulk. But you could put so many different movies, IPs (including DC) on that ride and still make it work and popular.
But when it comes down to it, if there was no other hurdles (like licensing issues with six flags and other companies) I am sure Universal could see some benefit to having DC over Marvel. Especially if they could somehow sell the Marvel rights to Disney for a high price, and get the DC rights at a better deal -- the end result could be very beneficial to Universal's bottom line. The problem lies in the other hurdles, and getting the right deal. That is probably not happening. But I guarantee those conversations have been had, just likely didn't go anywhere ("what do you mean you want 10 billion for that IP...")