There's two main reasons that it has come to this:
1. You can't call someone a liar to their face in a public forum. You just can't. Even if you have hard evidence that they're chock full of shit, everyone would gasp and you'd be accused of "vile personal attacks".
2. There is no objective truth anymore. Each side has a veritable army of pundits, fact-checkers, and think-tanks to bolster their argument and denigrate that of their opponent. So it doesn't work to call Romney a liar, because he can just pull out a study from the American Enterprise Institute that would support his argument. And then FOX News would run stories all day about how mean old Obama unfairly called Romney a liar, when very smart "non-partisan" experts at AEI and Cato and Heritage and places like that agree with him.
I think #2 is far more pernicious. And I don't see a way out of it. It's the crux of what Stephen Colbert's "truthiness" is all about: there's an entire industry devoted to creating a particular narrative and selling it as "truth", with different sectors for the various major political ideologies. And the public in general just takes a lot of that at face value, or even when they subconsciously know it's bullshit they will seek out sources which already agree with what they think, and with the other sources they read, so that it just creates a huge feedback loop and makes liberals more liberal, conservatives more conservative, and the two information realms they inhabit increasingly further and further apart.
Eventually they're going to be in two entire different realities. It'll be like alternate universes coinhabiting the same physical plane. In one, Teddy Roosevelt was a badass who embodied muscular foreign policy, American hegemony and was the living avatar of America's swinging cod. In the other, he was an anti-corporate, monopoly-busting defender of the poor. In reality, he was some of both. But that intersect is getting smaller and smaller by the day.
To borrow the old metaphor of the blind men and the elephant, all the blind men are lining up on opposite sides of the animal. One group sees nothing but the mighty tusks and powerful trunk, and the other sees nothing but a whole lotta ass.
I suggest that the next presidential debate have an army of live fact checkers validating every word said. And any factual discrepancy discovered will result in the electric dog training collars that the candidates wear being set off.
See Mitt Romney get funky in Debate II: Electric Boogaloo!