There are a number of different reasons for this principally
1. Aircraft cabin configuration where are the galleys located, seating configuration etc.
2. Airline preference
a. Fuel saving, aft C of G usually saves fuel although the actual saving varies by aircraft type
b. Speed of turnround, the operator might prioritise a quick turnround over fuel saving this may necessitate use of but forward and rear holds so two crews can load the aircraft.
c. Loading restraints may mean that cargo can only be loaded in certain locations.
d. Seasonal loads like Skis