Because...Although D'Antoni, Jackson and McHale aren't very good coaches, they're still 10x better than Corbin will ever be.
Jackson, maybe. Both are too early in their careers to tell.
D'Antoni? I guess he is a better specialist. He was given the perfect conditions for his system and still never made the finals. Since, he has been a below average coach.
McHale? **** no. Corbin is winning with Jefferson, something McHale could never do. The Rockets have a super-star and are below the Jazz in the standings. It's not like they lack depth or talent outside of Harden either.