You do realize that three rides opened within the last 6 years at WDW are princess based, right (SDMT, LM and FEA)? The most popular Table Service meal opened recently is also based on a princess (BatB). They replaced a RIDE with a Princess Meet & Greet. Tokyo just recently announced that they're building a Tangled boat ride and Hong Kong, Paris and Tokyo are all building Frozen Ever After. Moana is getting a show in Hong Kong. They are not shying away from princesses at all.
They're using other properties because they've pretty much used the princesses for all they're worth in most cases. They've also under-utilized properties like Star Wars and Marvel, etc. All they are doing is trying to make full use of all of the IPs that they have.