From a New York Times "political memo" headlined "Tax Cut Timing Proves Elusive for Democrats":
The economy's continued slow growth largely explains why ending those tax cuts, which apply to about 2 percent of Americans, proved easier said than done for Democrats. But other factors also explain their vacillation this year, including a crowded legislative agenda, the worsening political headwinds and, perhaps most of all, Democrats' chronic insecurity about dealing with tax issues.
That "chronic insecurity" explains it.