The Real Reason Cyborg Isn't In Titans
The DC Universe show Doom Patrol already has its fair share of surprises. But longtime fans of the comics, movies, and cartoons were especially surprised to see a familiar face join the DP's ranks. Victor Stone, famously called Cyborg, made his debut appearance in the Arrowverse during Doom Patrol's first season. This is significant because Cyborg doesn't typically hang around the misfits from the Doom Patrol. He's more famous as a member of the Justice League or Teen Titans.
The decision to put Cyborg in Doom Patrol is especially confusing when you consider that DC Universe already has a Titans show that predates Doom Patrol. In fact, most of Doom Patrol's cast (except for Cyborg) made their debuts in an early crossover episode of Titans. It stands to reason that the Arrowverse would follow the original character's origins and introduce him as a Titan.
Yet for some reason, the Arrowverse decided to go a different route with Victor Stone. Doom Patrol shows us a very young Cyborg, one who hasn't quite made it big in the hero game. He doesn't start out in the Titans and move to the Patrol or the League like one might expect. The Doom Patrol is his first super-team. He doesn't even know who the Titans are.
Cyborg isn't in Titans because they aren't even in the same universe
It seems like the minds behind Doom Patrol are doing everything they can to distance Vic from his Titan origins, and there's a canon explanation for why Cyborg never found the Titans: They simply do not exist in his universe (as far as we know).
The Arrowverse crossover event Crisis on Infinite Earths involves a whole mess of classic multiversal shenanigans. During Part 5 of the Crisis event, viewers are shown a handful of the alternate universes that exist parallel to the Arrowverse's main universe. As it turns out, the Titans show takes place in a universe called Earth-9. Meanwhile, the cast of Doom Patrol (Cyborg included) exists in Earth-21.
So, the Doom Patrol fans saw cross over in an episode of Titans is a similar, but ultimately alternate, version of the Doom Patrol that appears in their eponymous show. Story-wise, this means that the writers for both shows have more freedom to tell stories. What happens in one universe doesn't affect the other.
Unless, of course, DC decides to do another crossover. The walls between universes are notoriously easy to breach when it comes to comics. Earth 21's Cyborg might one day encounter Earth-9's Titans. Though, it's equally likely that both universes have their own versions of each other's missing persons. Regardless, fans who want to see Cyborg chum it up with the Titans once more can rest easy knowing that it's never an impossibility.