I've got two theories and they both probably have some merit:
1. Immersive Environments
When people are on vacation, typically they want to get away from real life. Disney, particularly the Magic Kingdom (and the early WDW Resort), is a great way to be transported to several different "places" over the course of a couple days without having to fly all over the world. You land in Orlando and in four days you can be in the South Pacific, the Old West, Turn of the Century America, Colonial America, New Orleans, the Caribbean, etc. This is almost impossible to accomplish anywhere else. Granted, imho, these environments used to be far better at transporting the guest to a different world (encroachments on the experience have slowly seeped into almost every corner of every land), but most of us are adults and people get hooked when they are youngsters.
Which brings me to ....
2. The Special 8 Year Old Theory
I have a relative who is a 68 year old man. He's extremely wealthy, lives in a wonderful house, has a large great family, lots of grandchildren. He travels often and is at a point in his life where he can do or afford pretty much anything he wants to do, when he wants to do it. How does he spend his free time? He watches westerns on AMC and professional wrestling; he'd watch it every night, all night if it's on. Now most people would say, "that's odd, most old men don't watch two men in a scripted fight and enjoy it." I admit, I thought it was odd for awhile.
When I made a joke about it once, his sister told me her theory. When they were kids, their family was pretty poor, but they were the first one in the neighborhood to own a television set. Back then, there wasn't much in the way of programming. But once a week, all the neighborhood kids would come over to their house and watch wrestling on the TV. Then they'd watch the western that came on afterwards. It wasn't often that he was a big shot, but on "wrestling day" her brother always was. She believes that people always love what makes them feel special or important when they are 8 years old.
Now, me? My parents both worked late during the week and I was basically babysat by my grandparents until 7 or 8 PM everyday while I was young. My dad worked most weekends, too - so I rarely saw him that much when I was very young - mostly for an hour or two before I went to bed. But every summer, we went to DisneyWorld. I had both my parents, all to myself, for a week - complete undivided attention. Those weeks were the highlight of the year, every year - nothing came close, not even Christmas. Memories of these trips are crystal clear. The photographs we took over the years there are some of my most prized possessions. In other words, DisneyWorld trips were my "pro wrestling and westerns". I bet WDW is subconsciously "pro wrestling and westerns" for a lot of people.