The reason for the relative success of BRS vis-a-vis CWL is the affluence of its catchment area.
Bristol has become a very successful finance and media-orientated city, and both those sectors are high-income.
Just to the NE is Gloucestershire and the Cotswolds - where house prices are high, there are lots of affluent retirees (with money, time and the inherited belief that flying ought to be more expensive than this), and where many Londoners have second homes.
South is Somerset/Dorset, could be described as 'Cotswolds-lite' in terms of rural affluence.
Bristol is the closest airport for all these areas notwithstanding poor road links, and few would consider Cardiff second-closest - people would opt for BHX or LHR in preference.
Contrast Cardiff/South Wales: still a lot of industrial decay, but with some grant-induced inward investment which provides low-paid manufacturing jobs. Other than small pockets, no seriously affluent areas, no aspirational areas (except Cardiff Docklands).
If the flight's not available from CWL, options are BRS and BHX. For those north of the Heads of the Valleys, BHX is probably as accessible.
I see more success for BRS in the future. The only hope CWL has is if Wales becomes independent with Cardiff it's capital. Now there's a thought....