Because the parks are seen mainly as a way for promoting the Disney brand.
Its the overall change in the company that began twenty years ago. Disney has shifted from being a primarily focused on creating products (Disney Feature Animation) and more focused on distributing products (ABC).
Just like movies are shown in a theater, the parks were thought of a stage to present Disneys immersive stories. Disney created original works to present on that stage Pirates, Haunted Mansion, Jungle Cruise. Each land within the park was a type of show you could see a western, sci-fi, an adventure story, a fairy tale. Direct movie tie-ins were located only in Fantasyland because Disney had already made so many fairy tales in movies. And this was also the decades before DVDs, since the movies were only released in theaters once every seven years, most guests to
Disneyland/Magic Kingdom had never seen several of the movies.
But beginning in the 1990s, the economics of the company changed. Instead of making most of its money from what it created, Disney began to make money from what others made and Disney resold to the public. ABC is nothing but a vast machine to resell programs made by outside companies. Outside producers like Pixar, Jerry Bruckheimer and Walden Media funneled movies through Disney Studios.
The economics of the dead guys company required Disney to produce something new if they wanted to earn more money, the new media Disney could increase revenues just buy selling you more.
The entire concept of synergy is that Disney can sell you the same product in many different forms and in many different places. You could go see
Toy Story. in your local movie theater. Then you could go to WalMart and buy the
Toy Story Special Collectors edition on DVD after seeing it advertised during the ABC broadcast. Your children can watch the Buzz Lightyear TV cartoon series on the ToonDisney channel while playing their Woody toys and dressed in their
Toy Story t-shirts brought from The Disney Stores. When you go on vacation you can be entertained by a
Toy Story show on the Disney Cruises and playing a
Toy Story video game on your Disney Mobile phone while waiting in line for the Buzz Ranger ride in the Magic Kingdom. I could go on, of course.
Its the new media concepts of brand extension and brand longevity. The idea is that people will buy a known product (a brand) over an unknown product. The more people buy just based on that brand, the more likely they are to buy that brand in the future. People will buy the sequel to an existing movie on DVD rather than buy a brand new movie for the same price. Disney believes that its guests would rather go on rides that they are know than go on rides that are based on a new concept.
And yes, Disney really does think youre that stupid and guible.
Its a safer bet when Disney builds a new ride. Disney thinks that anything based on Pixar has a better chance of being popular than a new ride Disney is convinced that a Pixar ride will push more plush, t-shirts, DVDs and trinkets (and thats all Disney thinks the parks are good for anyway).