The two don't necessarily correlate.
If you don't have enough people buying the book, you don't have enough people buying the book. That should tell you the direction you're going in messed up somewhere.
FYG went wrong for going to dark.
D&A may have gotten a little too high concept in places.
Waid really didn't do a good enough job at explaining what the Threeboot was trying to do with fixing the underlying flaws of their "utopia."
Johns wasn't as concerned with making Legion work on its own as he was making it a part of the Superman story he pretty much abandoned when he quit the book.
Levitz failed to fix the Legion's underlying problems (aging team, continuity issues, etc.)