Excellent question!
40 yr old Kim Jong-nam, the eldest son of the late Kim Jong-il --- was widely considered to be the heir-apparent to his father and the next leader of North Korea until 2001.
At 30 yrs of age Kim Jong-nam was arrested at New Tokyo International Airport, Japan, traveling on a forged Dominican Republic passport. He was held and then deported to the People's Republic of China.
He had been trying to visit Disneyland.
The incident caused Kim Jong-il to cancel a planned visit to China because of the embarrassment to both countries.
Kim Jong-nam fell out of favor and was later reported to be living in Macau,China.
30 yr old Kim Jong-chul is the middle son of Kim Jong-il.
Kenji Fujimoto, Kim Jong-il's personal sushi chef, wrote in his memoir, I Was Kim Jong Il's Cook, stated that Kim Jong Il thought Jong-chul was "no good because he is like a little girl".
I think that says a lot.
Kim Jong-chul has been reported in Singapore --- once this year while attending an Eric Clapton concert.
Kim Jong-un -- either 27 or 28 -- educated in Europe, speaks English, a military man, well trained in politics since he was a teenager, rabid Chicago Bulls fan, likes girls as opposed to be like a girl--- seemed the proper choice.