I can't remember the details, but perhaps it has to do with the aircraft type and jurisdiction as to how much weight can be carried in overhead lockers.
One of the recommendations from the Kegworth crash was:
Originally Posted by AAIB Report
The certification requirements for cabin stowage bins, and other cabin items of mass, should be modified to ensure the retention of these items to fuselage structure when subjected to substantially beyond the static load factors currently required (Made 30 March 1990).
If a country's aviation authority and airlines are to adopt these recommendations, then they have to do it pretty much uniformly (or possibly have one rule for mainline and one for commuter flights). You can't really vary it according to load or aircraft type. So, if one aircraft type in the fleet has a maximum overhead locker weight of 8kg/pax in a full plane, then that's the rule.