One important reason is that Romania was a satellite in the Soviet sphere of influence, which was collapsing under Perestroika and Glasnost. North Korea was and has been propped up by the Chinese government, which under Deng was successfully able to offer economic advances in lieu of political freedoms to maintain Party control, and ultimately, regional stability. The Chinese watched the Soviets with alarm and disgust, and were very careful not to follow in their footsteps. This book is also relevant: http://heinonline.org/HOL/LandingPage?handle=hein.journals/frdint19&div=12&id=&page=

This is a really interesting question, in part because it eventually turns around to feed on itself. Kim il-Sung and Ceaușescu were friends (or at least as friendly as two heads of state who don't see each other frequently can be), and the North Korean elite was pretty freaked out by Ceaușescu's death. There's a strong argument to be made that one of the reasons that NK has been so reluctant to liberalize in the post-Cold War period is because of the manner in which Ceaușescu died.

I don't know enough about Romania's experience to feel okay about making extensive comparisons, but I can tell you a little about what NK did to assure that Kim il-Sung remained in power, and why the country seemed oddly immune to the political and economic forces that capsized so many "Communist" governments toward the end of the 20th century.

Divide and conquer: Kim divided North Korean society into dozens of groups arranged by their degree of perceived loyalty to him. The formal system for this is called songbun. We don't know the exact number of groups that songbun codified (and the regime itself may not make any serious distinction between many of them), but they're broadly divided into the "loyal," "wavering," and "disloyal" classes.

The basics were simple enough:

• People in the "loyal" classes and their children (songbun is considered heritable) were given access to the best education, jobs, and homes, and were the most likely to be granted the enormous privilege of residing in Pyongyang. They were a minority of the population; I've seen estimates range from 5-15% of NK society.
• The "wavering" classes were a few rungs down the ladder with limited opportunities to move up, mostly in the form of military service or labor accomplishments. Most of the population was considered to be "wavering."
• The "disloyal" classes were rarely trusted with positions of any importance, and were even moved out of border towns for fear they might try to escape. There are a few exceptions here and there, mostly in cases where the regime wanted or needed something from a person or family they might otherwise have banished to the "disloyal" classes right off the bat, but they're not common. The "disloyal" classes constituted around 20-25% of NK.

Songbun is still active in North Korea, though perceptions of what truly constitutes a "privileged class" have skipped around a bit since the end of the Cold War. Previously "disloyal" classes like Japanese-Koreans or merchants who had access to foreign currency or goods were notably better off than "loyal" classes like run-of-the-mill government bureaucrats when NK started to struggle. However, the system still served (and continues to serve) a useful purpose from the regime's perspective. It decreased the unity of the population -- people with fewer privileges resented those with more -- and it marginalized the people with the most reason to feel unhappy.

Personality cult: NK has had decades of propaganda in schools, government publications, official histories, news programs, and media that carefully reinforced the depiction of Kim as the savior of the Korean people. Even if the more skeptical or educated citizens distrusted the more ludicrous portrait of him as a living god, he was still a very useful figurehead in an ultra-nationalist state that had been humiliated by decades of Japanese control. Add the government's management of entitlements -- all food was from Kim, clothing was a gift from Kim, everyone was given special foods and drinks and presents on holidays, etc. -- and you had a society that was divided against itself by songbun but united in its admiration for and loyalty to Kim.

Concentration camps and collective punishment: The camps were built specifically to hold people that Kim saw as either a threat to his power or as an unstable/uncontrollable element in North Korean society. The U.S.S.R. had its gulags and the Chinese had their reeducation camps, but the particular genius or horror of the North Korean approach was that the family of an offender would accompany them to the camps.

Lots of people can be brave when they only have their own necks to worry about, but it's a lot harder to justify taking a stand against a despotic government when you and everyone you love will go to a concentration camp with you if you're caught. The only mercy allotted to them was that the actual offender would typically go to a much harsher camp than his/her family members, and their spouses (on the rationale that they weren't related by blood) were spared. However, it was common to allow the family's release only after the offender had died.

Geographic isolation: I think the geographic element has a tendency to get overlooked. The Korean peninsula is bordered solely by China and Russia (and the Russian border is really, really tiny), and they were both content to leave North Korea's internal politics alone and enforce border control as long as NK served their interests. By contrast, Romania is bordered by Ukraine, Moldova, Serbia, Hungary, and Bulgaria, in addition to having coastline along the economically and politically valuable Black Sea. It is very difficult to truly isolate a country under those circumstances.

Small size: North Korea is almost exactly half the size of Romania. For the Americans out there, the most useful analogue is the size difference between Pennsylvania and Michigan. This, too, has a tendency to get overlooked; it's a lot easier for a dictator to impose his absolute will on a smaller country than a large one.

Ceaușescu's death: As we observed earlier, Kim and Ceaușescu had been friends, and the latter's death was a pretty raw shock. Kim had every reason to be afraid that that was the fate that lay in store for him if the regime ever liberalized. The enduring lesson of the Cold War's end for dictators like Kim was that liberalization might open your country to new wealth, but it was also terribly dangerous for an existing regime.

When people are struggling to get by and don't have much in the way of useful information about their circumstances, they just kind of muddle along. However, when they get the toxic combination of money and quality information in a state they believe could be doing better than it is, and is at least somewhat less likely to shoot them outright than it used to be, they revolt. I would argue that Ceaușescu's death continues to influence North Korean domestic policy to this day, because they're actually right to be afraid of this. And because propaganda has so heavily controlled and colored everything about NK's existence -- that the government is literally enforcing the will of the Great Leader, the savior of the Korean nation and hero of the revolution -- reform would be a tacit admission that maybe the Great Leader wasn't so great.

I'm a little rushed this afternoon so this is a less elaborate answer than I'd like to give, but I hope it helps somewhat.