Originally Posted by
Green Goblin of Sector 2814
I can't really say for sure on the Legion, but I can make a few educated guesses at least for the Titans.
I think a large part of why the Titans faced a decline was because DC stopped investing in that property as its own property. Now, that's not to say that DC editorial just woke up one day and said "the Titans are donezo as far as we're concerned." No, it wasn't that monolithic or sudden. I think it was just the culmination of a few things that took years to unfold.
For one, there was generally this attitude that, no matter what, no team was allowed to outshine or was "above" the JLA. Not the JSA, not the Legion, and not the Titans, even though the latter two outsold the JLA at the time. Second, starting at least in the 90s, there was almost an effort to take the Titans characters away from the Titans franchise.
That kind of became apparent when Wally West became the Flash after Barry's death. Now, even though Wally was still friends with all the Titans, he eventually had to join the Justice League because he was the Flash. And the Flash belonged on the JLA...right? I mean, when you think about it, Wally didn't really HAVE to join the League just by virtue of being a Flash. Jay was a Flash who was active, but he wasn't on the League. He was on the JSA and he stayed on the JSA. However, because Wally's ascension to Flash was seen as him "graduating" that meant that he had to "graduate" to Barry's spot on the League even if that meant leaving the Titans behind. It was something that fundamentally undercut the central thesis of Wolfman and Perez's New Teen Titans run: that these characters had come into their own and weren't beholden to their mentors; that they were their own franchise. And, for the record, I think Wally West is the best Flash and that his years on the League are awesome. However, it did divorce the guy from his friends and yeah, DC tried to balance it for a time by making him a member of both teams at the same time, but at that point, it was kind of like Solomon splitting the baby.
At almost the same time, in another move that followed the same pattern of undercutting the Titans, there was a concerted effort by the editorial at the time to separate Nightwing from the Titans so that he could be folded back into the Batman books. So, that's what they did, specifically around the time of the Knightfall story arc. Dick left the Titans, took over as Batman, and after Bruce's return got a solo Nightwing series operating out of Blüdhaven, Gotham's somehow-more-violent sister city. People I don't think realize that that was not just a happenstance occurrence because that's the direction the writers wanted to take Dick. It was a conscious effort by DC's editorial to fold Dick back into the Batman corner of the universe. And, like with Wally, those decisions might have been great to read for Dick as an individual, but you have to admit that taking away the Titans' literal leader undoubtedly did a lot to weaken the franchise.