Here's why Richard Dean Anderson stepped away from Stargate SG-1. The Stargate franchise launched back in 1994 with the blockbuster movie of the same name from Independence Day director Roland Emmerich. The story followed a team of soldiers and scientists who cross through the titular gateway to another world and it starred James Spader and Kurt Russell. Stargate was a solid hit but while Emmerich and producer Dean Devlin originally planned a trilogy of movies, this never came to pass.
Instead, the property found a new lease of life on TV with Stargate SG-1. In many ways, the concept was better suited to television as it gave the mythology more room to breathe. The show picked up where the film left off while adding new characters to the line-up, with the series ultimately running for ten seasons. It also spawned a few spinoffs, including Stargate: Atlantis - which featured an early lead role for Jason Momoa - and the sadly short-lived Stargate Universe. While the franchise was hugely popular in its heyday, there are currently no signs of reviving it.
One reason Stargate SG-1 became such a hit was a great cast, led by former MacGyver star Richard Dean Anderson. While he was technically the same character played by Kurt Russell, his take on Jack O'Neill was way more sardonic and he was never short of a dry quip. Anderson's charisma was a big part of what made the show work, but around season 8 the character was promoted to the base commander, which was a way for Anderson to cut back on Jack's role in the series.
By season 9 Jack departed the show completely, though he made occasional guest appearances on Stargate SG-1 and its assorted spinoffs, with his last appearance to date being an episode of Universe. Richard Dean Anderson initially cut down his role to spend more time with his young daughter, which was difficult with SG-1's demanding schedule, before he left completely. This was also influenced by health concerns, after decades of playing action-heavy roles like MacGyver and O'Neill had taken its toll on his body, which included several knee surgeries during his time on Stargate-SG1.
While he still made occasional appearances, Stargate SG-1 was never quite the same after Richard Dean Anderson's departure, though he definitely exited for the right reasons. The actor has also said he would be up for some kind of mini-series revival of the show, so long as it had a clearly defined beginning and end.