I recall reading that he later regretted his decision to leave because with Brandon out of the picture, the show didn't really have a "moral center" or lynch-pin anymore. He was the last of the Walshes left (90210 was originally a family drama about the Walshes from Minnesota being fishes out of water in Beverly Hills) and after that, it was harder to find a focus. I'm assuming that they brought Luke Perry back in those last couple of seasons to fill the void.

The real reason Jason Priestley left Beverly Hills 90210 (https://www.nickiswift.com/238293/the-real-reason-jason-priestley-left-beverly-hills-90210/)

Beverly Hills, 90210 was on the air for 10 seasons. Some of the cast members stayed for the entire run of the series, but the cast members who didn't caused quite the drama. Shannen Doherty infamously left the show after Season 4, never to return again. And then, there were other actors who "left" the show, only to return with guest appearances, à la Gabrielle Carteris, Luke Perry, and Jason Priestley.

Priestley's character Brandon Walsh abruptly left the season during the fifth episode of Season 9. The episode was aptly titled "Brandon Leaves." Brandon's character disappeared after accepting a job in Washington D.C., which should have been a way bigger deal than it was, because he was an original cast member and the show started with the Walsh family moving to Beverly Hills in Season 1. They didn't even throw Brandon a going away party! Instead, he became an occasional talking point as the other characters moved on with their lives.

In August 2019, Priestly talked about his exit during an interview with TV Insider, admitting, "Yeah, there wasn't much closure for Brandon. I think Aaron [Spelling, executive producer] always believed that I was coming back," which unfortunately didn't really happen. Instead of attending David and Donna's wedding, Brandon appeared in a video message (recorded on a VHS tape) during the series finale to share some remarks, which was just not the same. Clearly, Priestley didn't burn bridges with the series, but why did he decide to leave?