Because it was decided that the two teams would compete separately.
Originally, there were hopes for the two teams to not only march as one in the 2008 Olympics, but to compete as a single entity, a 'Korea' team. However, negotiations failed, and the two teams ended up marching separately as a result.
Since then, there have been several diplomatic incidents between the two countries, and due to the changed relationship, it is unlikely we will see them marching together again for the foreseeable future.
The link is deprecated, but it roughly says:
The flag was not used in the 2008 Summer Olympics in Beijing, China due to the decision made by the Beijing Organizing Committee for the Olympic Games (BOCOG) and the International Olympic Committee (IOC), that the two teams would enter separately. The two countries also marched separately at the 2012 Summer Olympics in London. North Korea did not participate in the 2014 Winter Olympics in Sochi, Russia.
The flag mentioned is the Korean Unification Flag, which looks like this: