The following article contains spoilers from Dark Crisis: Young Justice #1, on sale now.
Young Justice is something of an oddity in the DC Universe. It is so popular with fans, yet seems to fluctuate in and out of continuity. Even when they're remembered, there is still some part of the world itself that seems intent on ignoring whatever troubles the young team is going through. No comic emphasizes that more than Dark Crisis: Young Justice #1 (by Meghan Fitzmartin, Laura Braga, Luis Guerrero, and Pat Brosseau).
In the issue, when her teammates go missing, Wonder Girl goes to their various mentors and family members to try and get help finding them. Rather than immediately jump into action, these mentors and family members, brush off Cassie's concerns, chalking it all up to either teen angst or grief. It's a surprisingly flippant attitude they all had to the problems of the younger heroes, and one that demonstrates how little they actually pay attention to the next generation.
One would think, in the wake of the Justice League's death, that more potential loss would stir the other heroes to act. Instead, there seems to be a certain stigma associated with the team. The Flash summed it up best by rudely stating it was teen angst. In his defense, there is a lot of truth to that statement. Any teenage team of heroes is bound to have angst, and Young Justice has plenty of history with it. Be it from relationship issues, to betrayals of trust, or even just adjusting to each other's personalities in general, the team is rife with teen turmoil. That was part of the fun of reading about them.
Yet, this isn't the only thing causing people to dismiss their disappearance. Nightwing and Jon reasonably believed that there is a lot of grief going around, and that might have motivated the other members of the team to isolate themselves to process it. Even if that was what happened, during such an emotionally devastating time, it would still be a reasonable decision to check on the team. Losing loved ones can lead to dark times for the bereaved, so it would be in everyone's best interest to check up on Young Justice to make sure that they were at least emotionally okay. Even this concern is disregarded, however.
There is also a degree of hypocrisy in their attitudes. Each of the heroes Wonder Girl went to were all in her position at some point in their lives. Drowning in teen angst, emotionally traumatized, needing someone to check in on them to make sure they're okay. Nightwing and the Flash could have benefited from having more attentive caretakers, and now that they're the ones in charge they're making the same mistakes as their mentors.
Giving them some defense, they do have their own problems. Young Justice certainly isn't the only one grappling with grief and with the Justice League gone the remaining heroes have to pick up the slack and deal with the fallout. They may not have any time to worry about another problem. Perhaps they also don't want to. They've already lost mentors, parent figures in some cases, the loss of their younger siblings or children might be the straw that breaks the camel's back. Anything happening to Young Justice after such a loss might be enough to finally crush their spirit. Pretending everything is fine, might be the easier, if not more selfish, course.