Question for OP: How long ago did you visit? Because ever since Potter opened, Universal has been improving every day in all areas and they don't seem to be stopping.
Answer to OP's question: because both of their parks are fully fleshed out theme parks that offer more thrilling experiences than anything Disney can offer me. Because unlike Disney, they now care about the show quality of their attractions, and are still interested in wow-ing the public rather than just making something that is "good enough" or sometimes not even "good enough."
This.
This is the reason why Universal wins for me. I used to love Disney. Universal in the past 15 years has created so many mind blowing experiences. Spider-man, Mummy, Men in Black, Terminator (when it was new), Simpsons, not to mention the entire Potter thing. They've spent the money and pushed the technology on their attractions. Not only this, but they show they care about show quality through their awesome maintenance team and the Spider-man revamp. They take all these elements that are out there and put them together into one amazing experience with one goal in mind: To blow you away.
There are only two rides that have EVER completely stunned me as I was exiting: Forbidden Journey and Spider-man.
Within the same amount of time, I've seen Disney slowly slipping away into a "good enough" mood. And the bar that can be called "good enough" seems to be getting lower and lower. I fear that most people are blind sighted to this because, well, "it's Disney!"
When I went on Countdown to Extinction, I was terrified. It was an awesome experience that overwhelmed my hearing senses. When I went on it as Dinosaur, I thought it was still pretty neat, but not near as good as the original. I remembered a more aggressive profile. When I got off of it in December, I thought "?!" Half of the original effects weren't working, and the door to the time travel chamber wasn't working, so you could clearly hear the next scene. That ride was in horrible condition, and should not have been running. Then I found out it had been like that for months? What's the excuse?
As for Everest, why should I bother when the whole point of the attraction hasn't worked in years?
Don't even get me started on the Imagination Pavilion.
Midway Mania I think is a fine example of half-assing a ride. It feels like I'm just moving from TV to TV in a big warehouse to play a different game. I think it would have been really awesome if we could have had physical targets to shoot at in addition to the video screens. I will also NEVER understand why they have FASTPASS for this ride. It already has a low capacity and a slow moving line. Why add another line and split that capacity in half?
I do like Tower of Terror, however. It's the perfect combination of projections, physical sets, and thrill ride. You can't get that experience anywhere else. And the attraction is still in excellent condition.
And that huge blue hat? Yeah, that's a sin!
So after all these years, why is the Studios still a half day park? Why is Animal Kingdom still a two hour park? Why does Disney seem to not care so much anymore? They're making the money! Put it to good use, and stop with the nextgen crap. How is that going to help me when I visit the parks by myself and not staying onsite?
Universal I think does these things so much better. The employees seem to actually be happy to be there. Their smiles look real, not cheesy "I work here so I have to smile." Their overall guest relations seem to be genuine, and I have never gotten an automated response from guest relations either.
So there it is. I like Disney, and I want them to be better and get back on track. But I think Universal is always going to be better because of the current direction they're going.