Answer:
Because half filled sub levels are more stable than partially filled sub levels
Step-by-step explanation:
The reason for the exceptional configuration found in some atomic orbitals is due to the special stability of half-filled sub-levels over partially filled ones. 
One of the characteristic thing about filling some atomic orbitals we have to bear in mind that there is a stability conferred on an atom by being partially filled. 
The energy needed to remove an electron from a half-filled configuration or inert gas configuration is very great. 
This is why when there is a choice to have half-filled orbitals, it is desirous to go for this configuration.