I always enjoy reading MV's sensible, well-researched and well written posts
It has always been inevitable to me that Bristol would prosper in comparison to Cardiff due to their respective catchment areas. Bristol is a wealthy city, with many employed in financial services, the media and professions. It is surrounded by wealthy counties (Gloucestershire, Somerset and Wiltshire) where affluence leads to a higher propensity to travel than the average. These are counties that attract the affluent retired, again with a high propensity to travel.
Compare with Cardiff, which although the seat of Government in Wales, is in an economically depressed region. A cursory look at the average size and standard of housing unit will confirm this apparent generalization.
Finally, Cardiff Airport is disadvantaged by its geographical location. It is some 10 miles south-west of the City centre - away from centres of population by the coast. Fine from a noise point of view, but harder for surface access. I would imagine the travel time from a large town like Newport would be similar to CWL and BRS.
One sympathises with those who want CWL to prosper, but the reality is that the demographics favour Bristol. And that's why the airlines choose it over CWL.