The original concept was that it is better to stall the wing than to have it break, so before jet aircraft, the
Vb speed or severe turbulence speed was the square root of the designed g load of the wing which was 2.5 positive g (clean configuration); the square root being 1.58 times the stall speed. The square of the speed margin factor above stall is the g load in a condition severe enough to stall the wing. So, the
Vb speed varied for the gross weight.
With jet aircraft, that concept was impratical considering the narrow range of speed at operating altitude, so a design maximum gust was used. Considering the wide range of weights, a single speed is probably not the best answer. Lighter aircraft of the same type should fly slower to avoid excessive g loads.