There are definitely fads in fanfic, where certain types of storylines are really popular for awhile and then fade out, but one of the plots that has always been predominant is the typical "Sad girl with hard life who doesn't feel worthy of love meets Backstreet Boy, who somehow falls in love with her, teaches her how to love again, and makes her life worth living, until lots of drama (usually involving love triangles and/or babies) threatens to tear them apart" romance. There are different takes on it; sometimes Mary Sue has been abused or somehow traumatized in her past, and sometimes she just lacks self-esteem because she's overweight or "not traditionally beautiful," yet the Backstreet Boy manages to overlook all her issues and fall in love with her, baggage and all. It's the typical wishful-thinking fantasy fanfic. I think a lot of people started writing fanfic as teenies out of wishful-thinking fantasies, so it makes sense why there are so many like this. And some of them are good! Overused doesn't equal bad; plots only get overused because there's something appealing about them.