Access to the wing would be more complex; there's nowehere obvious to put a hatch. (Leading edge is bad for aerodynamic reasons; trailing edge is too thin; top is hard to access from ground; underside is accessible but the hatch is where you'd put the 'floor' of the cargo space, so you'd waste space for the hatch.
Wings are also a strange shape internally, both due to the external aerofoil shape and due to the load-bearing structure throughout the wing. It's relatively easy to put holes in ribs to allow fuel to pass, rather less so for baggage.
Fuel is also easy to distribute in a systematic fashion in the tanks; cargo would be subject to loading variations, with implications for e.g. flutter
You still have to put fuel somewhere, and fuel in the fuselage is a bigger fire hazard, so its also safer to put fuel in the wings and cargo in the fuselage.
A few reasons off the top of my head.