Why did Jean-Luc Picard leave the bridge of the USS Enterprise in USS Enterprise. He'd had countless adventures, and saved the entire Federation many times; he'd even encountered his legendary predecessor James T. Kirk, who had told Picard to never let them take him off that bridge.
And yet, in spite of that, Star Trek: Picard opened up long after Picard had led the Federation do just that. It revealed he had been promoted to iral, and his career had ended not with a bang but a whimper, when he failed to persuade the Federation to continue committing to the greatest refugee effort in galactic history. Star Trek: Picard season 1 was essentially the story of how a broken man picked himself up by his bootstraps and got back to saving the galaxy. But many questions were left unanswered, not least just why Picard had left the Enterprise in the first place.
Most of these questions are answered in Una McCormack's official tie-in novel, The Last Best Hope. This story opens on the bridge of the USS Enterprise, on the last day of Picard's captaincy. He gets a warning from Geordi La Forge that the Enterprise's scanners are picking up some very strange readings, but before he can investigate he's ed by Starfleet and asked to return to Earth for a priority briefing. When Picard arrives at Starfleet Command in San Francisco, he's greeted by iral Victor Bordson and then-Captain Kirsten Clancy. They told him the shocking news; the Romulan star was about to explode in a devastating supernova.
Picard was shaken to the core at the news, intuitively sensing the scale of the coming humanitarian tragedy. "It will demand the very best of us," he observed. "The best engineers, the best s, the best ships." At this stage, Starfleet agreed, and to Picard's shock iral Bordson revealed he had already chosen the best of them to lead the refugee effort. Starfleet offered the job to Jean-Luc Picard himself, although it would require him to leave the Enterprise and accept a promotion. Picard accepted, setting aside his own desire to remain on the Enterprise, believing this was where he could do the most good.
Of course, in the end the refugee plan stalled after a Zhat Vash who were implacably opposed to synthetic beings. And so Picard's decision to leave the Enterprise meant he had pinned everything on mission that was doomed to failure. Una McCormack's Last Best Hope simply makes this seem all the more tragic.