As for why the show is leaving, well that's because a Frozen themed live show is moving in. The new show will have "elaborate costumes and sets, stunning special effects and surprising scenic transformations" and numerous production numbers. Read more about both the retiring show and the new production from Disney here.
Taking over a huge portion of the park's backlot streets, the spectacle features millions of lights and decorations and is quite a sight to behold. The removal of the holiday tradition makes us wonder if the park's backlot areas will be removed sooner than later for the announced expansions.
It is always sad to see something like this end, however, the replacements that are in the pipeline will make it quite hard to complain!