The Seattle Seahawks didn't want to pay him. That is simply one of the reasons that the team traded wide receiver DK Metcalf to the Pittsburgh Steelers this offseason. Couple the future contract with the receiver wanting out of Seattle, and Metcalf had to go.
What is inarguable is that Metcalf was one of the most productive receivers in Seahawks history. He is big, fast, and strong, and he has a drive to be great. What he lacks is something that Pittsburgh and quarterback Aaron Rodgers have probably already learned, and will re-learn during the regular season.
DK Metcalf is a bad route-runner.