The following contains spoilers for Dark Crisis: Young Justice #1, on sale now from DC Comics.
Nobody can blame DC's heroes for being busy right now. Dark Crisis has just begun and, with the Justice League dead, the next generation is almost entirely unprepared for what comes next. They'll have to create their own team whilst dealing with the many increasing threats the League would have faced. Even still, that doesn't give them an excuse to forget an entire generation of heroes. If anything, the likes of Jon Kent and Dick Grayson should be going to them for help, however, none of them really cared about the Young Justice generation.
At least, that's how it looks in Dark Crisis: Young Justice #1 (by Meghan Fitzmartin, Laura Braga, Luis Guerrero and Pat Brosseau). Tim Drake, Conner Kent and Bart Allen (aka Robin, Superboy and Impulse), disappeared at the beginning of this new series that ties directly into DC's latest epic event. The only one who seemed to have even noticed was Wonder Girl, Cassie Sandsmark. Cassie gained a little sleuthing experience while playing detective in the "Trial of the Amazons" Wonder Woman event. She put those newfound skills to work tracking down her friends in this issue. She questioned every hero she could think of; shockingly, nobody knew what happened to them. They also didn't seem too concerned that an entire generation went missing either.
Although Jon Kent and Nightwing showed a little compassion when they told Cassie they wished that Conner and Tim were around to help, they still weren't willing to go looking for them. They deemed their current responsibilities too important. The same went for the Flash Family. They were too busy searching for the lost Barry Allen to search for his son, Bart Allen. None of them were worried about their supposed loved ones and just assumed they were either grieving in their own way or just off messing around.
This attitude towards these missing heroes shows a general lack of interest in their entire generation. Newer and younger heroes have replaced all the members of Young Justice. Dark Crisis is supposed to be all about the next generation stepping into the shoes of their mentors, but instead of doing so, they're straight-up skipping the literal next generation to the one that follows them.
Jon Kent replaced his father as Superman while Conner who has been around much longer is still Superboy. Tim Drake is currently sharing the Robin title with Damian Wayne. Although Wally West is back as the Flash, his longtime sidekick Bart has been replaced by Wallace West. It's also worth noting that Bart graduated from Impulse to Kid Flash before the New 52, but when Wallace came along he was demoted to Impulse again. Then, of course, there's Cassie, who's now sharing the title of Wonder Girl with Yara Flor. Oddly though, it's Yara who's being considered for the Wonder Woman mantle instead of Cassie.
There was a point in time when the heroes of Young Justice were believed to be the successors of the current Justice League. There have been multiple storylines that delved into alternate futures and shown just this. The most prominent was the "Titans Tomorrow" storyline in the early 2000s' Teen Titans. Although they ended up becoming evil, Tim, Conner, Cassie and Bart all took on their mentors' identities. However, they didn't go bad in every timeline, a few showed that they were more than worthy replacements.
This all makes perfect sense from a narrative perspective. Dark Crisis is taking some elements of the abandoned 5G and adapting them into a major event. Unfortunately, that means the Young Justice generation has to be sidelined and ignored. Giving them their own Dark Crisis tie-in series is a perfect solution to the problem. It delivers a different story, one focusing on the nostalgia surrounding these characters and how they can finally move forward to become a part of the world that ignores them now.