I think it's because the fandom exists partially so people can explore things which they know aren't going to happen. I mean, good grief, I know Draco/Ginny is never going to happen - but I love it almost more than Ron/Hermione, because it's got so much potential and so many possibilities. These two characters, both very real and with definite personalities - but they have almost no interaction in canon. Just imagine what it would be like if they did meet for lengthy periods of time - how would they react to each other - affect each other? Weave in the war behind that, the conflict between their families ... it's a hell of a lot more exciting than reading Molly/Arthur, wouldn't you say? I realise that this possibly makes my reading tastes shallow, but I enjoy the HP books for the adventure, the fun, the darker sides, the conflict, the endlessly wonderful fight between dark and light, the plotting - and that's what I look for in fanfiction, even if the basis of the fic isn't entirely canonical.

Also, to me, reading and writing Draco/Ginny uses my imagination so much more than if I read or write a fic about, say, Molly/Arthur. Molly/Arthur is already there, we know what it is - whereas D/G is an unknown quantity. It makes you think a lot more.

People like reading epic romance, angst and passion and a mention of the happily ever afters - not the happily ever afters themselves, because happily ever afters make dull reading. The majority of fans would prefer to read a good adventure featuring Draco/Ginny than a quiet, serious exploration of the relationship between Harry and Hagrid. There has to be something of excitement, conflict, and interest in a story, and frankly, Harry and Hagrid's connection is not very interesting except for essays.

Just my two Knuts. :)