It would clash with the zeitgeist currently prevalent in Hollywood. If they decided to pursue it, it would be from a "subversive" angle, as they like to boast.
I think there was a general sense the three wise men included people not of the jewish tradition. So although islam came along a few generations later it was implicitly baked into the birth scene, the prequel story. Kinda. I mean if you're a medieval artist, what're you going to go to, with a weird and wonderful magus boxing up some myrrh and frank, incensed cookies?
Archbish Hippo and Good old Augustine aside, I think most of the stone cutters in the gothic period didn't really get the idea of "people of colour" very well. Those black madonna generally started out white and .. age and fingerprints happened.
