The reasons for moving the date are many. Here are a few:
People will be home from school, which will free us from having to schedule the Megabattle during short school vacations.
The weather is more likely to be better. Now that we are hosting the Megabattle off site, moving/setting up for the Megabattle in a snowstorm would not be good.
There are lots of conventions that happen between the holiday season and the spring. Trying to plan around those can be problematic.
More time for everyone to build their armies and more time for us to work on the tables and terrain is a nice bonus.