Ain't it funny how a show like Marimite, which isn't even licensed has over 1,400 posts, while FMA, which is broadcast on a major cable network has only 69?
Keep in mind that the user base of danbooru is a very self selecting bunch. What appeals to the standard danbooru-er is only tangently related to what appeals to the normal TV watcher.
I wouldn't even consider danbooru to be a good guide to "fanart", too much sex.
Maybe it's because we're all pervs. FMA has a few girls with little sexual appeal, while Marimite is nothing but girls, all who are potential lesbians...except maybe Eriko.