Ah, but the Titans also have fundamental themes at their core that go to their identities as a team. They are the younger generation who will one day replace the old and who clash with the generation that came before them. And while the X-Men does a better job of sticking to their central tenant, that is still a pretty universal theme: one that seems to enter the national zeitgeist almost every few years. Think about all the animus that has been thrown the way of the Millennials or Gen Z the past few years.
You're addressing the symptom, not the cause. The X-Men characters can be popular as individuals...because they weren't mismanaged and didn't decline in popularity like the other two franchises. Though, I mean, I think enough people actually are fans of Starfire, Raven, Cyborg, and Beast Boy that you could say they're popular among the general public, if only because of the animated series from the mid-2000sAnother important factor is that, while the X-Men characters are a team, there have been several of them that have broken out and become fan favorites as individuals. Wolverine is the most obvious, but also Rogue, Gambit, Kitty, Storm, Phoenix, Deadpool, Cable, and others. No, most of them can't support a solo book for very long, but fans do like seeing them together and having writers explore their pasts.