How come North Koreans never rebel for democracy against the autocratic rule, whereas almost every other country in the world has successfully seen or is rebelling against the authoritarianism in some way or other. What makes Kim Jong-un so effective as a despot?
To some extent because the Kims have fully embraced being despots and successfully coopted their military in supporting them.
A lot of the successful rebellions, or at least protest movements, in the 20th century ultimately came down to the military refusing to shoot their own citizens at scale. People power sounds great, except it ultimately cannot succeed against an entire army willing to kill the masses.
The Kims have managed to convince a large proportion of their populace that they are in constant danger from the outside world and they have the Korean war to back up those claims (never mind that they started it). They've set up extensive secret police and informant networks, they've set up concentration camps and are not averse to filling them and they have a policy of punishing not just dissidents but also their family by association. They also run an extensive indoctrination program convincing people they're literally near-Gods (shooting stars on Mount Paetku when they're born, that kind of thing). Last, if you watch a bit of what's going on, they've been ruthless enough to kill even close family members to consolidate their power base. I assume that the military is structured and overseen in such a fashion to prevent any possibility of military rebellions. Without claiming precise knowledge, I assume some of the following are being used: quick rotation of officers through units, keep officers from serving together any length of time, using commissars, garrison troops away from home, use agents provocateurs to expose disloyalty.
At this point in time, protesting in NK, assuming you even thought that there was any reason to doubt Kim's wisdom, is a sure fire way to make your life much shorter and more miserable and anyone not actively repressing you when you attempt such a thing puts their own life and family in danger. Stalin and Mao both remained in power until their death, through equal ruthlessness, and had they had better succession plans they could have transferred power to designated heirs.
NK's system would indeed collapse the moment it showed any sign of weakness or humanity, but as long as it is sufficiently ruthless it can keep the Kims in power, which is all that really matters to them, even if the country as a whole is a miserable failure by any metrics, even including military capabilities.
I think it comes down to two (or three) issues:
Ruthless execution of anyone remotely perceived as a threat to Kim. Gulags (with a high mortality rate) for other offenses. There have been hundreds of executions since Kim Jong-un took power, mainly targeting the higher echelons of military and economic elites. Some were carried out abroad as assassinations. Without leaders, it's impossible for opposition to coalesce much if at all. (As an addendum, collective punishment is sometimes employed against the [extended] family of the [perceived] offender.)
The most extreme censorship anywhere in the world. All media is state controlled. There is basically no internet access in NK, except for a very small minority of elites and foreigners. Most of the country's intelligentsia (universities etc.) are limited to using the Kwangmyong, a tightly controlled intranet, air gapped from the (global) internet; the [Chinese] Great Firewall looks lame/tame in comparison. There's no way for mass protests to be coordinated as in other countries (no Twitter etc.) As an extension, there's virtually no contact with foreigners. Even the cellular voice phone networks are separated. The few foreign students that NK has (mostly from China) also are segregated to a good extenent. Nevertheless, some estimates based on NK defector reports suggest that one million North Koreans (illegally) listen to foreign radio stations. But they obviously can't organize much.
Somewhere on the continuum between 1 and 2 (and surely helping achieve both) is (again) the most extensive internal security apparatus known (quoting from the book How Dictatorships Work):
Sheena Greitens’s data on the ratio of internal security personnel to population in nine countries shows average ratios of 1:124 for North Korea (1:40 if informants are included), the most intensively policed country for which we have information, 1:5,090 for Saddam Hussein’s Iraq, and 1:10,000 for Chad between 1982 and 1990 (2016, 9).
- Very low risk of foreign intervention seeking regime change. Even before acquiring nuclear weapons and missiles, NK could threaten (and still does) the South with massive artillery concentration in the Seoul area etc. One Pentagon estimate put the death toll of a war with NK at 20,000 deaths per day. Also, some Chinese source fairly close to leadership said China won't allow external regime change in NK (without getting into details how they would prevent it.) Likewise, both China and Russia have "watered down" sanctions against NK at the UNSC. And even if the didn't do that, economic sanctions don't have a terribly good record at bringing about regime change.
The entire NK economy revolves around the military. So much so, the NK just relies on foreign aid to prevent food and medicine shortages. The people live in poverty. A better question might be why hasn't there been a coup.
Further, people have no access to information other than state controlled sources. Inside NK, there's nothing like social media to allow people to gather into large crowds (something all dictatorships fear). And the Kims have shown that with "the bomb" other countries won't risk covertly agitating from without for regime change.