I don't know why, but he keeps bouncing on his feet on defense and gets blown by bc he bounces one way his man goes another, or he just keeps giving room until he's all the way in the paint at which point guys get layups or draw fouls on him. He doesn't strip guys like Kyle does, nor does he play a passing lane to deflect a pass like Kyle. He's not disruptive, doesn't rebound as he should, etc.
He's probably still learning to play D the way Pop wants him to. His athleticism helps for blocking shots and I think he can be a better system defender than he is, once he understands the NBA game, but it helps a lot to anticipate a play and he doesn't have good instincts there.
Kyle just doesn't get enough credit for being a good defender and hAving good instincts for the game. He was one last season too and has started games and competed against starters without being a sieve.