Basically it is because after the half full point, electrons coming in will have to pair with another electron. Electrons are negatively charged and repel each other, so this is a relatively high energy process - ie. it's relatively difficult to do.
I'm not sure how in depth you want/have to go, but here is a more detailed explaination:
This is to do with the multiplicity of the state. If the shell is half full, the total spin (and therefore multiplicity) is at a maximum - all the electron spins are aligned. This means that the electrons tend to avoid each other - an often used analogy is cars going round a roundabout, if they all travel in the same direction, they tend to avoid each other.
When you add more electrons, the spins start to pair up. Basically, the cars coming in now are going the other way round the roundabout and often come close to each other. Electrons are negatively charged and repel each other, so having a state in which the electrons must get close is relatively high in energy. This is the basis of Hund's rule of maximum multiplicity.
Search for 'Hunds rule of maximum multiplicity', and 'Fermi holes' and 'Fermi heaps'