If you listened to Trump last week, how could not go away with the impression that Fox News is only into the debates for the ratings, and the money to be made from them. Without Trump, there would be little to no interest in the Republican debate.

Trump had a set of rules before he would go on O'Reilly. O'Reilly violated them in one of his questions.

To answer your question why there are not any Democratic debates on Fox News. Fox News is considered an arm of the Republican National Committee. If you don't think that this is a case., take a look at the war between Fox News and Trump.