It's possible to argue that he actually underachieved during his first season (with NFL talent all over the roster). Frankly, I'm not all that impressed by beating up on a bunch of sad teams from Texas, either - especially when BYU couldn't manage to win against a tiny party school from the Sunbelt, and the season ended with a bunch of sad "what ifs".
Some fans feel like BYU is stuck in a declining rut, with coaches that aren't good enough to take the team to the promised land but are often good enough to avoid getting fired. They believe Sitake isn't it, and want to move on sooner rather than later.
Remember how Bronco got run out of town because he couldn't meet the expectation of a 9-win (or better) regular season? Sitake has already reset expectations to the point where a 7-6 2019 campaign was heralded as a major turnaround...