Because the Public has a Liberal Bias
Interested in seeing how the Republicans will do in the CNN/You Tube debate format? Tough. Looks like many of them will be bailing. Like Josh suggests, I guess if the questions don't come from one of those Bush-style town hall audiences - you know, the ones where the "town" excludes everyone who fails a detailed screening process - then Republicans can't handle them.
It should be obvious by now that CNN is too afraid to actually allow a controversial question. But I suppose that for today's Republicans, mainstream is the new controversial.