Because his defensive playbook/playcalling sucks and his choice of rotation is terrible. The worse part is he actually thinks he's smart at D (so smart that he doesn't even need a REAL D coordinator, he just has his bro come and hang out). That is the biggest problem. All of D'Antoni's assistants (Phil Weber, Dan D'Antoni, Kenny Atkinson) minus Herb all got their fame from their stint in Phoenix where they had the worst D in the league as well.
O'Antoni sacrifices defense for offense every single game. This basically results in a loss unless our team is hitting our shots. Look at our FG% and rebounding in the games we won compared to the games we lost. If we're not getting second shots and hitting the shots we take we're always going to lose. The great teams are GREAT because they can win when they're not hitting their shots and the boards aren't falling their way because they have a coach that UNDERSTANDS defense and implements the correct rotations/playcalls...
In general, WE NEED A REAL DEFENSIVE ASSISTANT TO TAKE THE REIGNS OR FIRE O'ANTONI! Those are our ONLY two options. Everyone who is getting mad at Barkley for talking sh*t about us should stop. We need the media to get on board and to push out the fact that our D sucks SOLELY because our coach doesn't coach D. We all know it but most choose not to believe it matters or the fact that it's true. It is true. D wins championships. If we want a championship our coach needs to coach D, not just O. When is the last time you saw O'Antoni mad at a player over not fighting through screens, not boxing out, or blowing their assignment? I've never seen it and I watch every game through a microscope. The latter is not the same for a player or ref messing up on offense, he'll blow a gasket. I seriously think there's no hope for this guy.