A few possibilities occurred to me regarding the formerly best-selling Legion Archives hardcovers.
1) It's possible the fanbase for Legion, as you say, has diminished due to the many re-boots of the series over the last 3 decades. It's possible even that some of that fanbase has died off, literally. I mean, I'm 56, and that 1958-1970 era of Legion in the Silver Age even precedes me by a good many years! (At my age, I started reading with SUPERBOY/LEGION 197 in Sept 1973.)
2) That the strong fanbase for the early Silver Age Legion is unique to that early 1958-1970 era, and once they'd reprinted that portion of the Legion run, that fanbase might not have been strong enough for good sales in volumes of the later 1970's era, especially when it got past the Cockrum and Grell runs. There is a period between the last Grell issues and the first Giffen issues (1977-1982) where there weren't steady creative teams with a huge following. I'd still like to see them in a high-quality hardcover format, but just sayin'.
Yeah that stretch doesn't get a lot of love. Conway, Thomas and Ditko while being big names were not exactly producing anything beyond average for the day. I remember after it was re titled after Superboy left they were under cover at a space circus. It was two issues! Still there were some great moments imho. Blok became a member. The Legion miniseries while not the best art had a great surprise ending. And Superboy came back. Also some nice Perez covers and while it wasn't Ditko at his best it was still Steve Ditko doing the art for a bunch of stories.
