I've always figured it's because canon is full of gen, but rarely porn and not always a lot of romance (and readers like romance: witness the romance genre, a good chunk of the historical fiction genre, and the number of Great Literary Romances in mainstream fiction).
As for why people like to put canon friends into romantic situations: it's an unexplored possibility, and a lot of fanfiction is about unexplored possibilities. *shrug* I think incest fic is more complex, but it's not my thing so I won't speculate.
Personally, I like gen a lot and I write more of it these days than I used to, but it's much easier for me to find gen I like in pro fiction than romance I like, since I don't really enjoy the Traditional Het Romance or many of the other popular patterns in mainstream fiction. So I look for interesting and unusual romance in fanfiction.
no subject
As for why people like to put canon friends into romantic situations: it's an unexplored possibility, and a lot of fanfiction is about unexplored possibilities. *shrug* I think incest fic is more complex, but it's not my thing so I won't speculate.
Personally, I like gen a lot and I write more of it these days than I used to, but it's much easier for me to find gen I like in pro fiction than romance I like, since I don't really enjoy the Traditional Het Romance or many of the other popular patterns in mainstream fiction. So I look for interesting and unusual romance in fanfiction.