This is the cope, but the actual reason is Mike Brown is old-school/poor af for an owner and refuses to pay coaches to sit on the couch.
The media didn't want to admit Pete was probably a goner because everyone likes him and it was his first year with the team, but the Vegas Stadium prints money regardless of how good the Raiders are any given year and Mark Davis no longer has concerns about dead money on his accounting books. Pete, as much as I like him, as much as I think he's a HOF coach and will get all the accolades he deserves from the Seahawks, at least, in the near future, changed none of his frustrating habits in Oakland and, worse yet, the culture there seemed toxic and defeatist. That's probably not all on Pete, the current drama surrounding Maxx Crosby indicates to me there's organizational rot throughout the team, but he's a culture guy. It's his thing as a coach. You can't have people jogging routes, missing blocks, and visibly not giving a shit and stay employed as a guy whose job it is to motivate people.