Well-defined relationships between characters. Some dark moments. Some philosophical moments. Plus big robots and space battles. Not much, judging from its paltry presence as a basis for fanfiction.
People watch it, enjoy it, then move on to another series.
Which brings me to an interesting corollary. Let's take another look at the fan who starts writing fanfiction for a series they liked, instead of just watching something else. In order to write the best fics possible, they watch the entire series. Just to get the details right, of course. I guess that's all I have to say on this issue.