Here is an explanation:
"For the first time in nearly 40 years, the U.S. is not issuing a Christmas stamp this year.
The U.S. Postal Service - which is only 29 years old itself - announced in early April it is skipping the 38-year tradition in 2000, in order to "continue prudently managing inventories."
http://www.virtualstampclub.com/noxmas.html