Pokémon fans around the world rejoiced after Serena gave Ash his first kiss when she said goodbye at the end of the anime's Kalos adventures. Amourshippers, fans who suspected and hoped for a romantic relationship between Ash and Serena, were indisputably vindicated by the surprisingly heartfelt moment. However, Pokémon fans learned the hard way that they should be careful what they wish for -- this moment is directly responsible for Serena's continued absence from the series.
Pokémon takes advantage of its longevity by occasionally bringing back Ash's former traveling companions to temporarily join him on his current journey. Misty and Brock have made several reappearances, and even Tracey pops up now and again. May returned in Sinnoh and Pokémon Journeys has seen the return of Iris, Dawn and the entire Alola gang. Max, Cilan, Clemont and Bonnie have never rejoined Ash, but they've appeared as cameos or in special episodes set after they've parted ways with him. This leaves Serena as Ash's only traveling companion who has reappeared at all since her journey with Ash ended.
Interestingly, Serena is the only one of all the girls Ash has ever traveled with that he personally invited along. Their friendship has never featured any of the childish antagonism that his friendship with Misty is famous for. Serena truly respects and admires Ash for who he is, and the two have a strong bond. Considering how popular Serena is, and how popular her relationship with Ash is, it seems like it would be a no-brainer for the show to bring her back.
Pokémon Journeys is basically the franchise's victory lap. Many classic characters, like Gary Oak, have already had cameos, so it makes sense that fans are clamoring for Serena's -- long overdue -- return. At the time of writing, Dawn has just made her grand re-return to the series, having already come back once before during Ash's Unova campaign. That the series would opt to bring Dawn back a second time before bringing Serena back once doesn't bode well for the chances of it ever happening.
Pokémon Journeys is all about wish fulfillment, and what greater wish-fulfillment could there be than bringing back the girl who gave Ash his first kiss? Unfortunately, that famous kiss is precisely why Serena will likely never return to the series. If she were to appear, the subject of that kiss would need to be addressed. Pokémon is nothing if not consistent, even at the expense of Ash's character. The series is about adventure, not romance. Addressing Ash's and Serena's kiss would be too significant a detour from the show's M.O.
Bringing Serena back without addressing that kiss would not only be a disservice to the weight it brought to the story when it happened, but it would also unleash the fury of Amourshippers everywhere. Pokémon finds itself in the strange position of having a narrative thread preventing Serena's return that is somehow both too significant to address, and too significant to not address. Serena's post-Kalos limbo is truly a no-win situation for the series.
A character like Dawn can return multiple times because it was her journey that changed her. In Serena's case, meeting Ash changed her, so her return would require more emotionally intimate character interactions than a silly kid's show can afford. For better or worse, Serena's character arc is complete. Her return would either force the series to pivot away from its core tone or damage her legacy. At the end of the day, this is Ash's show and he's not ready for the maturity that Serena's return would force upon him. If Ash ever does grow up, and his journey comes to an end, he may indeed walk into the sunset with Serena. Until then, she'll have to stay in the fridge.