US Nationals is the culmination of a three-part qualifying process for singles skaters (two parts for pairs and dance) through regionals in October and sectionals in November. It would be disruptive to the schedules of a majority of competitors to move regionals any earlier.
Holidays in December would make scheduling Nationals during that month tricky.
They have to consider the availability of appropriate arenas and the way the television network they have a contract with (currently NBC) wants to cover Nationals.
If the TV network were enthusiastic enough about December broadcasts to offer a lot more money, that would be a more likely impetus to change the scheduling. But the other issues that would be effected by the change would still need to be addressed.