Why not reconciliation?

Ezra Klein explains why:

1) They already used it: You can't call an audible and switch over to budget reconciliation in the middle of a legislative fight. Reconciliation instructions have to be written into the budget for the year in which they're used. That means planning about a year in advance. When Democrats passed the 2010 budget, in April of 2009, they did include reconciliation instructions, but they were for health care and student-loan reform. And they passed that bill back in March. You can only use reconciliation once a year, so that's that.

2) They passed a rule making it impossible to use reconciliation for bills that increase the deficit: When George W. Bush passed his tax cuts through budget reconciliation, Democrats were horrified. It was the first time reconciliation had been used to increase the federal budget. So when they took back power, they decided to prove their commitment to fiscal rectitude by passing a new rule: Henceforth, budget reconciliation could only be used for legislation that would reduce the deficit. This provided Democrats with precisely no protection against the charge that they loved deficits, and it'll likely be changed back as soon as Republicans retake the Senate, but for now, a rule's a rule.

So it's not just that Democrats didn't write the tax cuts into the 2010 reconciliation instructions, but that they actually couldn't have written the tax cuts into the 2010 reconciliation instructions.