Because we play exclusively man to man, teams have figured out how to exploit our weak spots by switching til they get the matchup they want. Then they capitalize in most instances. We don’t have 100% locked in man to man defense. Somebody is always going under a screen allowing a wide open look from 3, or they get caught in the middle of the screen because the other defender didn’t switch allowing a driving lane. Many times the screener is rolling off and getting a good look because his guy didn’t adjust quick enough.
Bottom line is great man to man defense is doable, but it’s hard and it takes a special group to do it effectively enough to not allow exploitation of certain players.
It’s the whole reason we should zone more and pressure more since we have the players to do so. Hell, he couldn’t teach those either though, so never mind.