So they need more work to get it in orbit. I just thought, psychologically speaking, nuke(s) orbiting around the Earth, hovering overhead, ready to come down and vaporize you would be more terrifying to general public than some missile launched from a location inside N. Korea.
The ballistic approach is better than having a nuke in orbit, since you can design a specific ballistic trajectory for any target. If you have a nuke in orbit, it would take a lot of energy to have the nuke re-enter and it would need to have precision maneuvering ability to vector itself to the target. The orbit will constrain you to targets near your ground track and you may not fly over your target for many days.