Eh, I think it's useful to point out the elephant in the room when it comes to the "why" of people not ficcing Robin Wood.

Obviously, short of doing an Havard-style implicit association test in relation to Wood, we can't tell for certain one way or another. I don't think it's "unfair" to point out that race plays a huge factor in society today and that fandom often shows the effects of it. If people are made uncomfortable by having it pointed out, then...well, sucks for them.

As you pointed out, lack of fic devoted to a character suggests lack of interest. So the next question is why are characters of color consistently not interesting to the bulk of fandom? Especially in lieu of what random (white) characters fandom often does show an interest in. With one character, it could be a fluke. With many, you have a pattern where the common denominator is race. In that case, it's not unreasonable to conclude that some amount of aversive racism is coming into play.

Frankly, I think it would be more relevant to talk about why there are so few characters of color on Joss's shows in the first place.

That is another useful discussion, of course. However, I wouldn't say it's "more relevant" when the topic under discussion is "Why isn't Robin Wood written about more?"

It's hard to say. Just going by my perception (which is probably completely unreliable), Gunn gets more fic about him than, say, Lorne. But much less than Angel or Wesley. I actually thought Wes/Gunn would be a more popular slash pairing (as well as Angel/Gunn), but apparently Angel/Wesley is the big "thing" in AtS (along with Angel/Lindsay).