The Democrats killed it. The tax and spend Democrats could no longer keep it up, their spending exceeded even their ability to raise taxes:

“Democrats are committed to ending years of irresponsible budget policies that have produced historic deficits. Instead of compiling trillions of dollars of debt onto our children and grandchildren, we will restore pay-as-you-go budget discipline.”–Speaker Nancy Pelosi, December 12, 2006
Well, as Emily Littela, the half-witted Gilder Radner character on Saturday Night Live, would have put it: “Never mind.” Last week Congressional Democrats formally renounced their ballyhooed budget pledge to offset any new tax cuts with other tax increases or spending cuts. We’re delighted to see this false promise go, but there’s a larger lesson in this failure for the tax and spending battles of 2008.
Senate Democrats gave up on “paygo,” as it’s called, when they realized they lacked the votes to offset the $50.6 billion cost of protecting more than 20 million middle-class taxpayers from getting whacked by the Alternative Minimum Tax this year. They’ve spent the year floating all kinds of tax increases to make up the difference. But in the end they passed an AMT relief bill without a penny to pay for it. Paygo is now pay gone.