It makes sense for Alaskan Airlines to do that since they mostly fly west coast only routes and so I am sure Anaheim is very popular for them hence the smart idea of decorating.
Getting to WDW in Orlando, Florida - well just too many airlines fly into Orlando and possibly then fly on to other destinations so for any one airline to go ahead and decorate planes when many of the flights stopping in Orlando might likely be continuing on to another destination would not make much sense and give other passengers a sense that they are in some way, promoting WDW when in reality, they are not. If an airline DID make a deal with Disney World, well then, it would make sense to see some decorations as "the Official Airline of WDW" but I can't imagine WDW wanting to tie themselves down and possibly detract ANYONE from coming if that airline does not fly out if their region.
As ever, it's complicated!!
But well done for Alaskan Airlines!!