William T. Riker (Jonathan Frakes) was no longer Captain of the USS Titan in Star Trek: Picard and the reasons why he gave up the chair proves the key difference between him and Jean-Luc Picard (Patrick Stewart).
Star Trek: Picard season 1 is set in 2399, 20 years after Commander Riker left his post as First Officer of Picard's USS Enterprise-E to become Captain of the Titan. However, when Riker and Picard saw each other again in Star Trek: Picard, both Captains had retired from Starfleet.
Read more on screenrant.com