Most of the "non IP" attractions were ones created by Walt Disney and his original group of animators turned Imagineers, like Marc Davis, Claude Coats, ect... people who had 30 plus years experience making successful motion pictures before they got into creating rides. It seems like today the Marc Davis/Clause Coats types have been replaced by George Lucas/James Cameron/Stan Lee types. The Disney Company is simply trusting this new generation of successful film makers to create the attractions for it's current generation of guests. I think in part that's why they currently do things the way they do, because Walt Disney and his original guys are no longer with us, to not only to create the attractions, but also, Disneyland was a TV show before it was a theme park, and Uncle Walt and his guys would use that show to introduce and "hype" those "non IP" rides to people. The Pirates of the Caribbean and Haunted Mansion were already famous years before they opened, so famous that people freaked out when WDW opened without the Pirates of the Caribbean ride. My parents grew watching that show and loving it so much so they took me to WDW, and I loved it so much that I take my kids... so those "non IP" rides have a legacy and sentimental value that has built up over the past few decades and generations.
That being said, Expedition Everest is only about 12 years old, it's a shame it's broken, but it is a new generation, "original" ride, that seems to be pretty popular. The advantage of building a ride around an already popular movie IP is simply that they already "know" it's "popular", it's a "safe bet" to build something people already know and like, and will also probably buy something in the gift shop on the way out, and that is tough to argue with from a business stand point. I don't think it's completely out of the question, but I think the massive failure of California Adventure Version 1 probably scared them off from trying anything "new" for a while, and the success of Carsland, Avatarland, Toy Storyland and the anticipation Star Wars Land and Marvel Land is also pretty hard to argue with. It seems like at the moment they are "propping up" the "non Castle" parks with popular Movie IPs, because it's a safe bet that Frozen, Ratatouille, and Guardians of the Galaxy will get people in the door at Epcot. At some point there's no reason to switch gears and try something new again, and they now use the internet and YouTube to hype "coming attractions" the same way the did with the old Sunday night "Disneyland" shows... it just seems to me, that at the moment, they aren't taking any chances on anything "new" or unproven, and they are also trying to expand the "Disney brand" beyond the core group of fans who grew up with the old Sunday night Disney shows, because there's a lot more people in the world today who know who Darth Vader is than know who Mike Fink is... although I personally miss Mike Fink
