It's simpler to say "stick to your man" than it is to teach a guy to know when he needs to break off. It ends up being far more exhausting for the defensemen who have to move around a lot more than they would in a zone system.
It may be an explanation for why our top defensemen always seem to be more tired with less TOI than other team's top defenders
Just raises more questions on his coaching ability if he's not capable or willing to do it imo.