Musudan being only confirmed missile launched to make the flight path -- a secure alaska or remote Canadian post would be beneficial to north Korea without much challenge. An ample submarine fleet could provide the security. Needed for the transport and base launch pad for executing more accurate North American targets.
Even with Thaad and over decade long missile development program that was underway... north Korea would benefit from being a more mobile force. Mobile, securing launch pads and being more hostile with more resources nearby.