Seconding this - Arthur/Eames from Inception, for example, have maybeeee 5 minutes of screentime total and zero character development, yet they have a huge (and pheonomenal) fandom. But I do think you have a point - most pairings only really take off if they have a well developed canon or fanon, more male characters have that canon, and creating that fanon is a lot of work (that people tend to only do for male characters)