There are a number of reasons. First, the DPRK was not a Soviet satellite state in the same way members of the Warsaw Pact were. The North Koreans effectively took advantage of the Sino-Soviet split in order to gain the aid needed to maintain their regime. They essentially played off the insecurities of China and the Soviets in order to get what they needed. This is evident in 1962 when the North Korean ambassador to China is very critical of Khrushchev and notes that his "revisionism will revive capitalism within socialism," agreeing with Liu's critique of Khrushchev's anti-Stalinism. Liu had similar discussion about Khruschev's revisionism with Kim Il Sung in 1963. However, they also sought to maintain relations with the Soviets, notably resisting Chinese efforts to label the USSR as a detrimental great power of the sort that China considered as the "main enemy."

The importance of this is that the North Koreans were not exclusively reliant on the Soviet Union to bolster their regime in the way most Warsaw Pact nations were. When Gorbachev declined to support the regimes in Hungary, Poland, and the GDR it effectively spelled their end. North Korea had another option. However, the period after the fall of the Soviet Union was exceptionally difficult for North Korea, as China was able to cut its support of the regime knowing the Kim Il Sung had no where else to turn. This is where the longevity of Kim Il Sung and the cult of personality that surrounded him assisted the survival of the regime. He lived until 1994 and was able to use his personal influence to help the regime survive the difficult years after the fall of the USSR and ensure the transition of power to his son, Kim Jong Il. The "Juche ideal" helped with this, though the absence of aid sparked a significant famine during the mid-1990s. This led Kim Jong Il to increasingly demand western aid and seek to use his father's strategy by playing China off of the West. Many of the cases were North Korea acted belligerently in the late 1990s and early 2000s is traceable to their desire to extract increased aid. This is most evident in their agreement to concession on their nuclear program in exchange for increased aid in 2003 (obviously it didn't last).

TLDR: A combination of other-than-Soviet patrons and a strong cult of personality around Kim Il Sung helped the regime survive.