This has been going on from the inception of televised presidential debates with Kennedy vs. Nixon in 1960, whether it's Republican vs. Republican, Democrat vs. Democrat, or Republican vs. Democrat. It's simply because libs control the airwaves on which the debates are seen (with the possible partial exception of FOX).
Conservative candidates know they have to deal with the prejudices of the moderators and can effectively counter them with preparation by deftly steering their remarks away from the moderator's biased questions. It really isn't all that big a problem for candidates who do their homework, most notably Ronald Reagan vs. Jimmy Carter in 1980.
Republicans should have their own debates, stream them on you tube or something, we don’t need to rely on TV anymore.
Thanks for the ping!