i think it has something to do with the number of annual passholders and the population size of the immediate surrounding area of the parks. there are many more people who can make a day-trip out to
disneyland to experience something that was announced last week as opposed to wdw where many people have to plan months in advance to take a trip. its hard to have special limited events that are not announced in far in advance for a place that is predominately tourists, as opposed to disneyland which is predominately locals.
it's the same reason we have small world holiday and the horrible haunted mansion holiday (yeah i know some people love it, but haunted mansion is my favorite ride and i'm not a fan of nbc and i think 3 and a half months is a lil exessive for an overlay)...