NPD analysts believe the main reason is the high price of Mac computers:
Pricing of Macs versus other computers were likely a key factor in the November decline seen by NPD. The Journal cites Gene Munster, an analyst at Piper Jaffray, and others on "price sensitivity" among consumers:
Apple rivals like (Hewlett-Packard) and Dell offered discounts weeks earlier than usual this holiday season, dropping some prices by as much as 50 percent. Mr. Munster said since last December, the average Windows PC price is down 35 percent to 45 percent; in contrast, Apple has offered only modest discounts of 5 percent to 10 percent on its PCs, analysts said.