That or a salary increase. Only those two reasons why a player would decline the player option. He is valuable now. There are lesser point guards in the league who are healthy and he is one of the best choices of any team who doesn't have a playmaker because he can score on his own, handle the ball with care, and he can make a play too. It's hard to find guys who could do both.
Players who reject a huge offer likely believe that they can still make a lot of money compared to the offer on the table. VanVleet was the leader of the Raptors last season, and his stats were very consistent. He can certainly contribute that to other teams as well. He desires a max offer, and with the Raptors having a new coach, there's no assurance that the current system will continue to work in his favor.