I think it may have a lot to do with who is perceived as being in power as indicated by who is seen more frequently on TV and is more visible making statements in the News. While Obama did win the election, we've since had the rise of the Tea Party on the Right, Pelosi losing the Speakership, and the prominence of several very vocal candidates running for the Republican nomination. No one on the left is really making news and we don't really hear about anyone except maybe Harry Reid and an occasional verbal stumble by Joe Biden. No one really sees Hillary anymore, as she is overseas all the time developing larger ulcers, I am sure. If anyone IS making speeches and statements, they are the ones in the news and giving fodder to the late night comedians and it spreads from there.
We have had Democrats like Rod Blagojevich and Anthony Weiner in trouble and in the News (and although it has been a while, how about crazy James Traficant?), and SNL regularly picks on Obama and sure had fun with Rahm Emanuel before he left the White House, but who else -Democratwise- is on the average person's radar? Also, who got made fun of more than Bill Clinton?
On the other hand, people running for President get scrutinized as they get deeper into the race and more of their dirty underwear gets dragged out into the daylight. And they make campaign speeches about 18 hours a day, so they are bound to start saying some stupid things, even if they are not stupid individuals. And they can be caricatured more easily when certain facts come out. Or when certain unfortunate and intriguing word choices are uttered.
I mean, how can the Democrats even compete for comic airtime with the likes of Newt "Moonbase" Gingrich, Herman "I don't have facts to back this up, but..." Cain, Christine "I am not a witch" O'Donnell, Rick "Ooops" Perry, Mitt "Corporations are people, my friend" Romney, Rick "Santorum" Santorum, etc., etc. The field is currently overcrowded on the Right.
.... and as soon as we get more Democrats back in the public eye (especially next time we have a gang of candidates calling each other names and throwing their own mud in a Presidential Democratic Primary, we'll see more humor again at the Democrat's expense.