This article by the Los Angeles Times points to the fiscal cliff as the reason:
“Don’t worry if you haven’t received your expected IRS tax refund. Most people haven’t — and the fiscal cliff is to blame. The Internal Revenue Service is far behind its normal pace in processing federal tax returns and mailing billions of dollars in refunds, according to a new report. Through Feb. 3, the agency sent out only about $4.3 billion in refunds, according to the analysis by Nicolas Colas, chief market strategist at ConvergEx Group in new York.”