Because some people are more interested in responsible policies for the future of this country, and not only concerned with keeping their own taxes low at all costs.
This famous chart allows us to estimate how much the average person in each income bracket stands to "save" if all cuts are extended (look at the Republican plan. Get used to looking at the Republican plan as long as Obama is president). Notice, people making under 100,000 dollars a year get, at the most, $1,871 a year cut from their tax bill. For reference, that's 36 dollars a week. If you make less than $75,000 per year, that weekly number is down to 21. If you make less than $50,000 per year, the tax cuts are saving you 17 dollars a week, and as you go further down, the number continues to shrink. You get the point.
Now, look at the top income bracket. Someone who makes $1,000,000 dollars or more per year gets a yearly tax cut of $103,835, which equals $1,997 each week. So in other words, if you make less than 100K per year, these tax cuts give you less in a whole year than they give to millionaires in one week.
I know in tough times like these, every little bit helps for many families, and losing even those modest tax cuts could be painful for some middle and working class people. Ideally I'd like to extend the tax cuts for those people. But in the long run, extending the cuts for the rich is far more harmful than raising taxes slightly on the middle class.
The tax cuts for people making $250,000 and higher will cost 70 billion a year (middle class tax cuts have already been accounted for in paygo, so they're deficit neutral if the extension passes and if there's no extension, we have a few hundred billion in undesignated revenue sittin around) , so even if we assume they are let to expire in 2 years, that's 140 billion deficit dollars of deficit spending. Now everyone who knows me knows I'm not opposed to deficit spending right now, as I believe spending can help energize the economy. But tax cuts for the rich are a wasteful and inefficient way to stimulate the economy. So I'd be glad to just lose the extension for the rich and not borrow an unnecessary $140 Billion.
If no extension passes for anyone, and we now have what would amount to a few hundred billion per year, I'd really like to spend that on public works and other legitimate stimulative measures but, realistically, the Republicans would never let another big stimulus bill pass. So if we can't use the money for middle class tax cuts and we can't use the money for job creation and investment, I say fuck it, go ahead and start paying down that big bad scary national debt. Austerity hawks have been warning that the economy continues to lag because our high National Debt makes would-be investors too nervous to make a move. I guess if a public works program is completely off the table in the next session, and we therefor can't spend the money on something smart, like re-vamping our infrastructure, we might as well see if puttin a little dent in our debt actually makes a difference or not.
Bottom line, the tax cuts for the rich passing is much worse than the tax cuts for the middle class failing.