I mean you are comparing SF which is not even considered a premium resort by universal to one of the most expensive resorts at Disney a fairer comparison would be Caribbean beach resort or port New Orleans
You asked why resort only stays aren't more popular at WDW. My answer is to say the Orlando area has a slew of nice hotels at better prices. I'm comparing those specific two because (I think) SF is a nicer hotel than CR. And those two just popped in my head.
SFalls is much newer, has a much better pool slide, a much better pool, a better arcade, better poolside activities, better outdoor dining, comparable free shuttle boats, better parking, and arguably better dining/bar options.
I've stayed at both hotels a number of times. If I'm not going to go to the theme parks, I'd choose SF over CR, even if they were the same price. The main selling point of CR is that it is next to MK. (things like broken elevators, etc.) The garden wing very much looks like 1970's office buildings. For a hotel-only vacation, CR is a bit of a depressing choice.
Alas though, I also mentioned Portofino. If I only planned to visit the hotel, and my choices were the WDW deluxes or Portofino, I might choose Portofino even if the prices were all the same. Portofino has a great layout, great rooms, nice pools, decent food options, great shuttle boats, and is easy walking distance to Citywalk.
For reference, I've stayed at nearly every WDW, most Universal, and many offsite hotels. I dislike the layout of CBR and POR, though it has been some time since I have stayed at either one. The new tower at Coronado is nice, but the food options outside of the tower leave much to be desired.
If they were priced the same, and it was a short stay, I might choose a tower club room at CSR over Portofino.
I just came back from a short stay at the new Endless Summer. I gotta say, for the price, it was AMAZING! The suite room layout is really nice. The only small drawback was it took a while for the shower water to get warm on our first day. The food court could also be a bit bigger, but they serve fresh berries as an option, which is nearly impossible to find at any WDW hotel, at any price point. For the price, I was very impressed with how nice the hotel is.