Coaching defense is a mindset. Playing defense in basketball requires concentration, anticipation, and quickness. If you watch the best defensive teams in the country, you will notice they react immediately to the offensive player's move, and stay with him every step of the way.
On offense, you're basically one step ahead of the defender, because you know what you're going to do. Coaching players to play effective defense, especially players who are used to being the best player on the floor regardless of the opponent, can be difficult if not impossible.
Prohm talks a lot about being a tough-minded defensive team. I haven't seen that so far, which makes me wonder how effective he is at teaching it. Kids know how to shoot and score, that part is fun. Defense isn't glamorous, but it wins games, doesn't require a home court, and is portable.