The space shuttle was untested in flying over the new year between December and January. It needed a clock reset and they weren't about to test that on orbit.
I assume they would have gotten around to testing it if the space shuttle program ever got anywhere near its planned operational capacity.
But with only 4 operational shuttles, ~20 days maximum ndurance and only 5-10 flights per year, it was simply enough to just avoid scheduling any mission in the latter half of December.
(Though, I notice STS-103 departed for an 8 day mission on December 19th 1999. That was cutting it close)
