Maybe the Y/N is often white because shock many fan fiction writers are white? It's a cultural thing - it's more acceptable to write fan fiction in most European cultures or cultures with European backgrounds. I've seen a few hispanic Y/Ns for that reason.

Plus, most of the stories I read are in...English! English, the language of the White, Anglo-Saxon, Protestants. Yes, English was spread all over the world and is the most commonly spoken tongue but most people prefer to write in their native tongue. And yes, people of all colors and ethnicities live in the United States and speak English - however, when you look at demographics, you are going to see white followed by hispanic being the most common "races".

Really, to me, this entire thing comes off as racist. "How dare the author write about a girl from their native culture!?" As most fan fiction writers are....white & female. Not all - but certainly an overwhelming amount. Which is why Y/N is white and female.