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The most obvious explanation is that Siskos character was left in a very different place than the other captains, figuratively and literally, at the end of his series. While Kirk, Picard, and Janeway all ended their series as captains and later became Starfleet admirals, Sisko ended Deep Space Nine not even living on the same plane of reality as Starfleet. He joined the Bajoran Prophets, aka the wormhole aliens, becoming a non-corporeal, extra-dimensional being existing outside of linear time. This makes it a lot more difficult for him to pop up in a cameo, unlike, say, Janeway, who was able to quickly video-call Picard to give him orders in Star Trek: Nemesis.

The only direct reference to Sisko in modern Star Trek had Lower Decks Beckett Mariner confirm in the season 3 episode Reflections that Sisko is working hard in a celestial temple. This presumably means that he is still Propheting with the wormhole aliens at the time that episode is set (2381 CE, 20 years before the currently-airing Picard season 3). When Lower Decks visits Deep Space Nine in the following episode, Siskos baseball is still sitting on Colonel Kiras desk, so he has clearly not come back for it yet at that point. In Star Trek Online, Sisko convinces another Prophet to give the player character a task, suggesting he is still at the celestial temple in that continuity as well.

But the celestial temple is hardly an impossible place to come back from. In fact, actor Avery Brooks specifically requested that Sisko promise his pregnant wife Kasidy that he would come back to her. Brooks did not like the representation of a Black man abandoning his pregnant wife to raise their child alone, and as a result Sisko appeared to reassure Kasidy that he would return presumably, while their child was still relatively young. As Picard season 3 has clearly shown, suddenly meeting your child as an adult is not the same as raising them.

Indeed, in the non-canon Star Trek novelverse, Sisko has already come back. In the Deep Space Nine relaunch novels, he returned on the day of his and Kasidys daughters birth. In the new series of comics from IDW, he returns after three years. In the hypothetical new episode sketched out by Deep Space Nines writers in the 2019 documentary What We Left Behind, Sisko reappears after 20 years and says, Im sorry, Jake. I lost track of time.

Even if Sisko were still with the Prophets by the time of Picard (which would break his promise to Kasidy, who would have been waiting 26 years by then), theres nothing stopping him from featuring in any of the current shows as a Prophet, just as Wesley Crusher made a cameo appearance as a Traveler in Picard season 2. In fact, he should theoretically have a very easy time showing up whenever he wants, since time has no meaning to the Prophets. Like Wesley, Sisko could jump to other timelines, and could even pop up in the 23rd century of Strange New Worlds or the 32nd century of Discovery, as well as the 24th and 25th-century-set Lower Decks, Prodigy, or Picard.

So Siskos return is far from impossible. But fans know better than to get their hopes up too much at this point. After all, Deep Space Nine tends to receive less attention among the wealth of references, homages, actor cameos, and other nods in the currently airing Star Trek shows.

Read more:
Will Star Trek: Deep Space Nine's Benjamin Sisko Ever Return to ... - Den of Geek