The answer is Rush Limbaugh:

My gosh, does nobody on this panel remember that we're running against Obama?…. Fox wants these people to tear each other up. Cause they want approval from the mainstream media, cause that's what the mainstream media would do….. You never see the Democrats pitted against each other, not like this was.

Yeah, dude, that's the nature of the debates and primaries.  The candidates have to disagree with and score points on each other before they go on to the general.  It's agreed that the ultimate opponent is X, but if you can't get past persons Y and Z who generally agree with you in a debate, you're in all sorts of trouble.