The Somewhat Positive Reasons
1) I really like the universes my favourite pairings inhabit. Take them out of those universes and you take away much of the appeal. Law enforcement, space travel and ::guh:: staying alive via swordfighting are interesting occupations. Ordinary professional fields, academia and trades are not. I've been there, done that and don't want to read about it.
2) The sheer physicality of my favourite characters is a huge drawcard for me. (*) I don't think this is especially related to the actors - it's related to their occupations. The more crucial it is that the characters be physically fit and skilled just to stay alive, the happier I am (think Pros and Highlander). Give those characters a job without a strong physical component and you've lost my interest. On the other hand, a wholly-physical occupation doesn't work for me either. Absolute top of my list for boring occupations: acting and being in a band!
3) If I want to read about characters in an original setting, I can go to the library. There are thousands of interesting novels about relationships between men. I don't even have to go as far as the library - I can just go to my bookshelf, and read Brokeback Mountain and the Hornblower books and Lonesome Dove and even Wilbur Smith over and over again. (OK, the last three are short on sex, but that isn't a problem for me - a subject for another day.)
The Not-So-Positive Reasons
1) I think I might be a fairly lazy reader in that I usually find it tedious to absorb the details I need to know to follow a story. When I'm reading novels, I almost always find the first few chapters a real chore. (I've just finished The God of Small Things on cricketk's recommendation and it was a hard slog through the first 75 or so pages. How are Chacko and Ammu related? What's the difference between Baby Kochamma and Kochu Maria? How does Margaret Kochamma fit in? Who is Chacko, again?)
Fanfiction generally circumvents that for me, since I already know who everyone is and how they got there. As soon as you make it an AU, though, I have to study what you're saying and memorise all the basic facts. (Yes, lazy, as I said.) I can't just go straight to the enjoyment part.
2) I feel disconnected from the "fandom" experience while reading an AU. There isn't as much common ground between the reader and the writer. Getting an highly imaginative story in return doesn't make up for this.
* This explanation was working perfectly until last July, when I discovered Sport Night. At this stage I'm still calling SN an anomaly for me - a fandom with no "physical" component.