Newsarama has a lengthy excerpt from TwoMorrows' "Back Issue" magazine #22, an interview with Paul Levitz and Keith Giffen discussing their work on the Legion title. The excerpt pretty much covers how they got to be working on the book.
LEVITZ: ...I was a real Legion fan as a kid, and [writer Jim] Shooter dropped it right at a point in time when I was starting to be allowed to write on a serious basis at DC. Whether I was ready to or not is arguable, but at least the tide had changed in that direction. There weren’t a lot of guys lined up wanting to do it, and I probably would have killed anyone of my sort of stature who tried because I wanted it that badly—and did it that badly.
SCHWEIER: Why do you say that nobody was really interested in doing it?
LEVITZ: Well, if you don’t love the Legion, it’s a pain in the ass.
GIFFEN: Yes, the amount of characters was so big that we’d always keep flow charts.
LEVITZ: Yeah, I mean, you’d literally have to write the book with a scorecard and notes, and that’s if you knew them and loved them. If you didn’t know them and love them, it was a lot of work.
GIFFEN: If you really didn’t care about these characters, it would be agony.
The magazine can be ordered through the TwoMorrows website if your local comic shop doesn't carry it.
The forum discussion after the interview is interesting too.