Short answer - they caused it.

Longer answer - Kim Il-sung did what Mao before him and Stalin before him had done and built a "cult of personality" around himself. History was rewritten so that every major positive event was because of Kim and his family. People who opposed him were purged and erased from the collective memory. History was re-written again so that minor positive events were also because of Kim and his family. Anything negative was the result of American imperialism or South Korean puppets or Japanese colonialism, all things which the Dear Leader fought against.

The reason Stalin and Mao's cults have collapsed is lack of control or heirs. Khrushchev and Deng Xiaoping were not descendants of their predecessors and they were both reformers, surrounded by people who did what their bosses told them to do. Kim Il-sung, on the other hand, played China and the Soviets against each other for the maximum benefit of the DPRK, purging anyone who stood against him and/or stood too closely to China or the Soviet Union at the wrong time, and he handed control over to his son, Kim Jong-il, who handed control over to his son, Kim Jong-eun.

Further, China and the Soviet Union are massive countries with multi-ethnic populations that have multiple, diverging needs and wants. Controlling thousands of miles of border with sometimes hostile neighbors and hundreds of millions of people is an exponentially greater task than controlling a few hundred miles of border with a friendly neighbor and less than 25 million people.

A brief note on your question - I would not say that the people of North Korea have a "religious love" towards the Kim family. Yes, some people are brainwashed enough that we could use that term. The majority of people though are simply smart enough to know that they must show their loyalty lest they get whisked off to Yodok or some other dark corner of the regimes security apparatus.

Check out Under the Loving Care of the Fatherly Leader by Bradley K Martin for an exhaustive look at the cult of personality built up around Kim Il-Sung and Kim Jong-il.