In part because when Kim Il Sung assumed power, right of the bat his "revolutionary lineage" was touted as a reason for him to be the leader, his father and grandfather and great grandfather all were said to have been revolutionaries (his great grandfather is told to have been leading the attack on USS General Sherman in 1866). This is unlike say Stalin or Mao, who rebelled against their fathers and family and became revolutionaries. The population was then divided into classes based on their revolutionary past, with children and grandchildren of collaborators and landowners bcomeing the new lower classes and the offspring of the revolutionaries and Korean War veterans being the new priviliged class. Thus, the most qualified person to rule would be from Kim Il Sungs family, seeing as he has the family history and pedigree and the revolutionary history (a past that was, needless to say, greatly embelished).

Kim Il Sung also placed his children in positions of power, with Kim Jong Il becoming "Head of the Organization and Guidance Department" in the Party, leaving him with a solid power base. I am unsure if Stalin ever did anything like this to his sons?