I didn't go very in-depth, but Howard has played for three teams below league average in DRtg.
I don't watch the Pelicans very much, but I'm going to go with
1. Poor coaching. Defense is 1/3 physical talent, 1/3 knowing where to be, and 1/3 working hard. They have the first one and the second two are coaching.
2. Severe lack of depth. The Pels are basically running with a 7-man lineup. A roster full of really good, well-rounded players can pull that off, but the Pels have too many offense-only players. With endurance being a scarce resource, guys are going to save their energy for what they do best. For the Pelicans, that means offense.
3. Davis and Asik are their only players with a track record of good defense. Anderson has been adequate, everyone else has been pretty bad over the course of their careers. Some of that can probably be blamed on playing for bad teams, but it also means that none of the players know what it is like to be a big part of a high-performing defense.