Fuselage Fuel Tanks
What specific regulatory (and/or practical industry) restrictions are there on the use of fuselage fuel tanks (located outside of the cabin pressure vessel) in transport category aircraft?
I've seen FAR Part § 25.963, which constrains emergency landing impact/scraping specifically for fuselage fuel tanks and FAR Part § 121.255 which constrains fuel tanks that are in designated fire zones. Are there other restrictions?
I'm curious why the transport industry has such sparse use of fuselage tanks. I certainly understand the advantages of reducing the wing bending moment as to why most of the fuel is traditionally located in the wings -- but I'm surprised there are not more fuselage aux tanks. Such fuselage tanks seem to be more common in longer range business aircraft (Learjet 60XR comes to mind).