That's kind of the point, though. Each side has to have a moderator they selected who shouldbe biased towards the candidate that selected them.
That ensure that both candidates get asked tough questions.
That would be unworkable though in a field of say 10 candidates, any six of which might make it to the debate.
Better for everyone to agree (or majority by vote) on moderators that are universally thought to be both tough and fair-minded... which I believe is the functional norm in primary debates. Or something close to that effect.
That the moderator(s) bags on one, or certain ones, is a show of bad journalism, to be sure, and certainly unfair. But outside of avoiding that moderator in the future, I really don't see how that can be changed without the quality of journalism rising by an order of magnitude - Which is really the problem at hand.