He showed big potential after his rookie contract, got a good deal. He has shown his extreme defensive weakness, he is aging, and not as valuable to teams as he once was. He just isn't worth that much money right now, especially with all the money becoming more and more tied up in stars. You could find very similar contracts for players following his exact same career projectory.
All that said, I absolutely agree with Kanter and think he is one of the true OG, real guys in the league. I appreciate his honesty, candor, and his ability to stand up for what he believes. He deserves major praise IMO.