Heat rises, psu at top will be sucking in all the warm air from inside the case, psu at bottom will be sucking in the air from the cooler lower part of the chassis, therefore the psu wont have to work so hard to stay cool and it allows exhaust fans to be mounted in the warmest part of the case, the top.
This is the theory anyways. :lol:
I thinks it allows for better cable management with the psu at the bottom although on some psu's it means the connectors need to be put on upside down which can be awkward. Also depending on what case and psu you have the psu cables can restrict airflow so top or bottom can help with cooling.