I think it may be a safety issue. In some areas those cotsworlds-severn barrows are proudly displayed, and if they were chalk capped, gleaming white and visible for 10s of miles. Maybe in those areas the builders of the barrows felt safe, it was their heartland, the barrows proclaimed their ownership in a way.

Whereas on gower, perhaps they were immigrants with a potentially unfriendly population so they felt the need to hide the barrows...

Just a (probably unprovable) theory :)