I think these two posts taken together make up a good point.
Comics are just extremely dense and lore heavy and kind of hard to break into these days, probably more than ever before, and they have much more to compete with as well!
The Legion are maybe doubly hard to break into, since their "corner of the DCU" is even more separated from the rest than most other characters' corners are. Making everything more interconnected, like having more crossovers between the Legion and the JLA, wouldn't necessarily help with that though. Yeah it might drum up interest, but another, maybe bigger, function is that it just makes the lore more convoluted.
I like Hypestyle's idea of doing concurrent mini-series of distinct adventures in which to focus on certain smaller groups of characters, so as not to overwhelm new readers with the Legions' like 30+ member roster.