I imagine that a holiday overlay for the attraction, is a bit more involved than hanging some tinsel & pre-lit garlands from the rafters of the show building. In order to bring this to the game experience, there will likely have to be a whole new show programmed, which will cost a lot of $$$ and time from the team that wrote the original ride programs and annimation. I seriously doubt the annimation programs came with a drop down list of scenery options. A separate program would have to be written, rendered, compiled, and tested to accomodate a holiday overlay that goes beyond physical props.
The up side is the ride could stay online while being written, and could be tested during the 3rd shift. Then its just a matter of loading the new ride program overnight to switch gears. No need for shuttering a ride for an extensive physical overlay.
The question is, when would Disney spend the capital for the programming team to write new code and annimation for the ride?