Why?
My best guess is that it comes back to the obvious: the two parties are basically the same. They are mediums of power for two elite groups that have divided up a bloated USG among each other. The leadership of each party knows that this symbiotic relationship cannot persist if a third party emerges and Ron Paul, who has rejected party dogma and in doing so mobilized a third way, is a threat to the status quo. This is bigger than the GOP.
The Democratic Party is vested in their incumbent president. Unless Obama kills a hooker on national TV in the next fifteen months, he is their preferred nominee. That much is clear because he has done just about everything else to lose the party's confidence. Mismanaged economic polices? Check. Backed down on social policy? Check. Changed stated plans on foreign policy? Check. Failed to resolve budget and debt issues? To quote that chick from the reality show, the one with the kid who got knocked up, "you betcha!"
Aside from getting Osama (a true political victory) and his healthcare program (a true political victory if you drive a Prius), he does not have much to run on.
But the party sticks by its leader even when popular support fades. Just Wikipedia some random Soviet republic- chances are they still have or have only recently lost their commie leader who remained in power years after the USSR collapsed- thanks to his party apparatus.
But the open secret in Washington right now is that Obama is vulnerable and might very well face a challenger. That challenger could be from inside the party, but more likely, because of the institutional barriers to challenging an incumbent, will be from outside.
A true left-wing, third party challenge to Obama would be a threat not just to his second term, but to the status quo in DC. Can you smell the fear on the Hill? Neither party wants to see a viable third party candidate. And Ron Paul's continued success encourages such a threat.
Ron Paul is a conservative Republican. Unlike Rick "I Supported Gore" Perry or Michelle "The Constitution Allows the USG to Regulate Homosexuality" Bachmann, Ron Paul has been steadfast in his beliefs for decades. But his unwillingness to play in a system which regularly violates the Constitution but greatly benefits the bureaucracy is untenable to the fat cats at the top of the GOP (Some of them are quite fat by the way- anyone wondering why Gary Johnson was not at the debate? My theory is Newt ate him!). It also scares the heck out of the lefties too, who fear his success in Iowa and other places might spark a liberal candidate who actually believes in what they are saying and stands by those beliefs.
That is why even leftist elements of the media are so desirous to dismiss Paul.
The ultimate proof, of course, is that Ron Paul would be, by conventional wisdom, the perfect Republican nominee if you are an Obama supporter. He is not the polished, smooth candidate presidential contenders have always been in the past. But as much as the Democrats fear Mitt and his hair taking over the White House, they are much more afraid of a third party coming in and ruining their shared monopoly of political power.
I hope we do see a primary challenge to Obama, or even better a left-wing, third party candidate. I agree with Ron Paul on his ideas, but also because he believes in something, not just being elected. It would be refreshing to know that Americans who do not adhere to a libertarian philosophy have their own choice. I know I am relishing mine.
When I go to buy soda at the grocery store, there are several dozen types to choose from. Of course Coke and Pepsi are the big two, but sometimes I pick up a Sprite or even a Fanta- just to keep 'em honest.
We should do the same thing in Washington. Fanta might not be the most popular soda, but it's good to know it's there when I want it.