There are a few reasons he gets away with it. One is that the Republican party is out of its mind. The party is so delusional that it doesn't think its lies are lies. In relation to that, it has media outlets (FoxNews and talk radio) that present and reinforce those lies as truths. Thus the voters tend to believe the lies and they expect the leaders to agree with them and then further reinforce those lies. It's all a big feedback loop.
The party itself and the conservative media are strong forces. But Cohen, via Steve Benan, says that the conventional media is also part of the problem:
"Romney gets away with it because he and his team realize contemporary political journalism isn't equipped to deal with a candidate who lies this much, about so many topics, so often."
I'm not sure exactly what this means, specifically what tool the media lacks to combat Romney's lying. I guess it could mean they don't have the ability to reach conservative voters who trust their reporting to the point where they would question their own leaders about their lies. I would agree with that to a large extent.
What I also think the media lacks is the ability to be as combative towards exposing lies and the truth as aggressively as people like Romney are at spouting the lies. At some point Romney is going to have to do an interview or debate at a place other than FoxNews. Not to mention that any reporter can follow him on the campaign trail and ask him why he constantly lies. Granted, it's not likely you will get an answer since he has no intellectual integrity. But at least you would be making the effort.
As Glenn Greenwald consistently points out, the media seems much too deferential to people in power. Romney should be an absolute joke and completely non-credible. But for the most part people just kind of brush the lying under the rug and accept it as part of the process that doesn't need to be challenged. The only way that changes and the Republican party stops being delusional is if they suffer some consequences for their intellectual dishonesty. And the media has to have a big hand in making that happen.