Can't comment on the t-shirt thing, but flags... it might be the case that no one has actually made a flag, or that no one flag has gathered enough support to become a ruling flag.

When someone said "why don't asexuals have a flag?" the idea had no doubt been raised before, but it took that one person to actually pull their finger out, rally people and now we do have a flag. In contrast, the genderqueer flag was one person who posted on their own website saying "this is my idea" and for some reason it kept coming up in google search and stuff, so people thought it was the official flag and that's how that one got momentum. It might be a case of "if you want one, make one".