I'm not as surprised as you, mainly because my previous big fandom (before LoM) was SG-1 and there's *so* many AU fic, it's scary. Mind you, it's not as "real" as Pros and the show even has an element of AU in it (what with the Quantum Mirror AND the episode where they all work as firefighters...)
A lot of the high-AU Pros fic (aka the stuff I won't touch with a barge pole) is old though, so I'm not sure it's an excuse in having to branch out. The Hunting series by Jane (one of the infamous elves AUs) is circa 1986, Arabian Nights, in which the boys don't even have their own names, is also from 1986. I could name lots, actually. It looks more like a general trend away from taking characters you know well and putting them in any situation to writing about the meaning behind scenes or reactions. Or, indeed, just torturing the characters for fun. (But as if I know *anyone* who would do such a thing...)
Re: Not quite on topic but...
A lot of the high-AU Pros fic (aka the stuff I won't touch with a barge pole) is old though, so I'm not sure it's an excuse in having to branch out. The Hunting series by Jane (one of the infamous elves AUs) is circa 1986, Arabian Nights, in which the boys don't even have their own names, is also from 1986. I could name lots, actually. It looks more like a general trend away from taking characters you know well and putting them in any situation to writing about the meaning behind scenes or reactions. Or, indeed, just torturing the characters for fun. (But as if I know *anyone* who would do such a thing...)