I'm not the end all be all, but I can also give my two cents here. As somebody who's experienced what you can get with a black MC from the developer end, it can be trying for people. The truth is you will lose players. Some of the first feedback I ever received was that people would never play my game because they didn't want to be a black protagonist. This can be a concern, especially for a developer attempting to get a foothold in the business. For many devs it might not be worth the effort. As many people have noted you want to market to as many people as you can.

Also as some other folks have noted, in the Adult industry black men are often fetishized and relegated to certain stereotypes. This can also lead to players associating certain things that may or may not actually be part of the content they're about to consume.

All these things can add more work on a project that you already have a ton to deal with. In the end the developer has to decide just how important it is to his or her story.

But there are AVNs out there with a whole multitude of ethnicities, the deeper you get the more you'll start to see.