Because it is convenient for the opponents of Christianity who run the mainstream media to identify it with the lunatic fringe. And because one has to be slightly unbalanced to want to start a media empire in the first place. There are such things as Catholic media empires, of course, but they have a slightly different character - look at the different profile of Mother Angelica, founder of EWTN, as compared with Falwell and Robertson. In general, the better the Christian, the less the effort s/he makes to draw attention to him/herself. When a good Christian becomes prominent, it is ordinarily because of extraordinary talents - take C.S.Lewis or the current Pope - rather than by any pursuit of popularity.