It's hard to mount an uprising when you're starving to death
The North Korean military is well fed while much of the general population is starving. It isn't hard to see where the military's loyalty lies. Also, the military has all of the weapons while the populace is reduced to eating bark. That's not exactly a winning formula for an uprising.
It also points out that the old saying "no country is more than 3 missed meals from a revolution" isn't accurate.