DC once had an old mandate that Clark be the sole survivor of Krypton and while it's since been lifted, the belief that the number of Supers in any given story should be kept to minimum seems to have remained.
Conner and Kara are a good example as due to the mandate she wasn't allowed to be brought back for a large portion of her early years after Crisis on Infinite Earths (not even by Greg Weisman himself when he pitched DC the idea of bringing her back) and Supergirl was completely rewritten. No longer was she Superman's cousin or even Kryptonian (100% or otherwise). Instead over the years, she was a man-made, shapeshifting lifeform made of synthetic protoplasm named Matrix, a human woman who merged with Matrix named Linda Danvers and the future Kryptonian-human hybrid daughter of Lois & Clark named Cir-El. When Kara finally did return, Matrix/Linda and Cir-El were shelved but Conner died before he and Kara were even given a chance to interact. Then he was revived much later right before the New 52 came along and reset everything again.
New 52 Superboy and Supergirl were essentially new characters but did form a bit of a bond near the end. But eventually New 52 Superboy was retired and rectonned-out and the old classic Conner was brought back with him and Kara once again meeting for the first time ever.... but now that the Dawn of DC has low-key shelved Conner again in order to push Jonny Kent becoming Superboy and the chances of Conner and Kara getting to be a family is once again on pause.
Even In a universe where they both exist, Earth-49 (Injustice), they never meet. In the Injustice 2 prequel graphic novels, Kara meets Blue Beetle instead of Conner after AMAZO's defeat. Conner doesn't even appear in the Injustice 2 video game.
In Earth-16, for all current four seasons of Young Justice, it's just been Conner and Clark. Then Conner ended up in the Phantom Zone, making The Light aware of the Phantom Zone Kryptonians. They had Klarion capture all of them and place them in stasis cells on the Warworld, except one, which he handed over to Apokolips as tribute. Guess who? Kara. Now, even though Young Justice is one of Max's Top 10 most viewed shows, it has yet to be renewed while Kara's with the Female Furies. Finance is SO weird. And because WB has no money to spare (and possibly because of YJ: Phantoms' critical reception not being as high as the first three seasons), we have yet to see a DC universe outside of the comics where Kon-El & Kara Zor-El meet.
TLDR: A bunch of cosmic reboots, editorial policies, lack of continuity in the Injustice series, the Young Justice series renewal status being in limbo and the Discard and Draw nature of how DC handles the Superfamily's members has stopped the Superfamily from ever really becoming a thing.
