When Star Trek: Deep Space Nine debuted on Jan. 3, 1993, then-Commander Benjamin Sisko was tasked by Captain Jean-Luc Picard to bring the planet Bajor into the Federation. When Deep Space Nine ended seven seasons later, Bajor remained an independent system despite being approved for membership in the planetary alliance. This was not an oversight, but rather an intentional choice on the part of the storytellers. Deep Space Nine was the Star Trek series meant to break the franchise mold, and Bajor never joining the Federation was a big part of that. Was it a mistake? In the series premiere, "Emissary," Sisko is assigned to the titular station and, after discovering a wormhole and a new alien species, became the titular emissary to Bajor.
Source：https://www.cbr.com/star-trek-deep-space-nine-bajor-federation/
