That is the Senate rules. Read this Daily Kos diary -
http://www.dailykos.com/story/2010/6/20/876730/-Filibuster-reform:-you-need-it-every-day
Well, let's start with the reason it didn't pass before Memorial Day as originally planned. The House took care of its part just before the holiday, and sent it on to the Senate. And as I said, you'd normally see a bill like this pass pretty quickly. But the political calculus of the Republicans is that any legislation passed by a Democratic Congress and signed by a Democratic President is a "win" for the Ds, and in their zero-sum world, therefore a "loss" for the Rs. And they can't have that. Or at least won't allow it without a fight. So they fight tooth and nail to delay and derail everything they can. Even if they can't ultimately kill a bill, delaying it gives them a chance to snipe at it, and as a consolation prize, deride the Democratic Congress as dithering and indecisive. How's that strike you, by the way? Criticizing Dems in Congress as slow to act... for having had to wade through the procedural slog of shutting down Republican filibusters, of all things!
So when this bill made its way to the Senate just ahead of the scheduled Memorial Day recess, the Republicans needed only to find some sort of excuse for throwing the roadblock of the filibuster up in its path. So they went with the old standby of being "deficit hawks" (despite having been noticeably AWOL on that score with regards to, oh, say... eight continuous years of off-budget war). Truthfully, it hardly matters whether there's an actual reason behind a filibuster. After all, it's been ages since a filibuster was actually about the thing which is supposed to literally define what a filibuster is -- that is, "extended debate." When was the last time you saw any of the people who insisted on voting against cloture actually show up to listen to and participate in extended debate?
At any rate, the fact is that the mere threat of a filibuster -- especially when the Senate is heading into a holiday recess -- is often enough to derail a bill temporarily. That's because even a bill that has overwhelming support in the Senate, and has more than enough Senators willing to vote for cloture, still has to actually go through the torturous and time-wasting process of invoking cloture. These days, Republican obstructionists are routinely filibustering even the motion to proceed to consideration of a bill, meaning they're filibustering the question of whether or not the Senate should even begin debate!
That means that getting to a vote on the tax extenders bill, even if you have 60+ votes ready to go, means filing a cloture motion (along with a petition signed by 16 Senators, which is never a problem), but then waiting for a full calendar day to pass after the day on which you filed the motion. Then, after one hour of session has elapsed on the second calendar day following the filing, your cloture motion is "ripe" for consideration. But remember, even if you trounce the opposition, all cloture gets you is the right to end debate... after another 30 hours of it! So now you've spent about a day and a half to get to your cloture vote, won the vote, but may have to sit by for another 30 hours before you ever get to actually vote on whether or not -- get this -- to begin debate on the bill!
Note that is just the process to open up the discussion on the bill - he goes on to explain that there is a cloture vote to close the debate before you vote. This happens both on the Senate bill and on the "conference" bill.
So, where they are now - they need to get 2 more votes that get 60. In the last vote we didn't have Lieberman or Ben Nelson (and Byrd did not vote - likely because Reid knew the count.)