I found this explanation in the Tribune archives very helpful:

From “Sunset breaks can flame out, cast lasting glow,”July 8, 2001, by  Kathy Kristof, Tribune Media Services

Tax sunset provisions have been around since the mid-1980s, when Congress created a formula for speeding so-called reconciliation bills.

Reconciliation bills have to conform with tax and spending levels already approved in earlier budget resolutions. That often requires putting strict time limits on the bill’s provisions. In return, reconciliation bills can’t be delayed by a filibuster or loaded up with unrelated amendments–tools that foes often use to kill pending legislation.

In today’s closely divided Congress, reconciliation protection allowed the then-Republican majority to pass the new tax law in record time. The trade-off was a landmark piece of tax legislation that could disappear in a decade.

The situation is particularly jarring with the new tax law because it affects all taxpayers and virtually every portion of the tax code. Sunset provisions used to involve a handful of narrowly targeted breaks, like research and development tax credits and tax breaks for adoption.

Nonetheless, sunset provisions do have some supporters.

“I think having a sunset provision is a good way to get rid of something that has completed its purpose or is a bad program,” said Danielle Doane, director of government relations for Citizens for a Sound Economy, a group that advocates less government.

Despite the furor that sunsets have caused in the planning community, it’s not clear whether Congress will allow these new tax breaks to expire.

“We are not worried that any of these provisions are not going to exist in 2011,” said Dan Danner, senior vice president for federal public policy at the National Federation of Independent Business, a small-business advocacy group. “We think most of them will be made permanent long before that.”

That’s wishful thinking, counters Martin Sullivan, an economist who once worked on the staff of Congress’ Joint Committee on Taxation and now writes a column in the weekly magazine Tax Notes. “It is going to be tremendously difficult to extend this bill,” he said. “Congress will have to figure out how to pay for it again and how to get the votes again.”

 The Senate lacked a 60-vote supermajority, but using this “legislative trick” (that they railed furiously about in 2010) the narrow Republican majority managed to pass the tax cuts (with 58 votes). One of the two Republicans voting no was John McCain.