Here's why we're going to go over the "fiscal cliff" in January. First, remember that this agreement is already a compromise between the two political parties. Everything that will happen was specified (and agreed to by Congress) by the Budget Control Act of 2011. This was the best Congress could do back then, and since the voters saw fit to return Congress basically intact (Republican House, Democratic Senate), there should be little expectation of much improvement.
Also, even though this "fiscal cliff" has already been approved by Congress, most voters don't realize that. Nor do they hold Congress responsible. Most political pundits still hold their opposing political party at fault (either party), not their members of Congress who are still doing the same old, same old.
Republican lawmakers want the fiscal cliff to happen because it includes significant cuts and changes to Medicare, something they have been unable to accomplish directly, but now they can achieve with fiscal austerity. For Medicare, Republicans lost the ideological battle but now stand in a position to win the war.
Conversely, Democrats see themselves as the responsible adults in the room. Based on the math alone, the tax rates must return to something more equitable and realistic. It is impossible to cut enough from spending alone. Without more revenue, important social programs would have to be cut. Democrats think they can accomplish this without being seen as complicit in this matter.
The real losers in all this are, again, the middle class. We will all pay thousands more in taxes, without fundamental problems ever being addressed. Corporate welfare and tax breaks will still be over $90 billion a year, and an individual taxpayer making $50,000 will pay 18 percent more, while a taxpayer making $200,000 will pay 13 percent more. Same old, same old.