Why has north korea's economy lagged behind south korea's?

Primarily economic sanctions. They are also a communist dictatorship, while South Korea is a booming capitalist democracy. A long series of bad decision making on the part of their leadership. Winch include things like government organized drug rings and grow ops, in place of normal farms. They boast the worlds fourth largest military (mostly undependable due to food logistics).

 The military are also the few people in the country who are regularly fed. Civilians and especially people in small towns and work camps aren't so lucky. 
A while back they enacted the 'lets only eat 2 meals a day' act. On top of all that, recent weather problems such as floods and droughts have put huge stress on an already inadequate agriculture model. U.N food shipments labelled 'a gift' are rounded up and by the government and then sold to their own people. Most countries and agencies are getting to frustrated like Redcross etc and are choosing not to get involved.