Cardiff and VoG are well off.
However, the Valleys area is poorer than parts of Poland, Czech Republic, Hungary and Romania in that it is eligible for EU structural funds.
With regards to GVA you're looking at 124th and 136th out of 139 NUTS3 areas for the Valleys and 34th for Cardiff and VoG. For Bristol you're looking at 11th, then 24th for the wider Bristol area, go further afield to Glos. it's 32nd and Swindon is 9th.
Until the Valleys area gets richer CWL will struggle, an airline making a choice of location will almost certainly look at things like GVA or GDHI of the airport catchment area. Then it'll look at where passengers come from to use the airport and for BRS they'll see that a significant proportion of passengers come from from Wales, but for CWL the proportion of passengers from England will be less. Airlines are businesses if they think they can make more money at BRS that is where they'll locate.
Vueling are doing very well because they understand what their target market is - people who want a holiday in Spain. They also understand that there is a market for that in South Wales. Strip away all of the typical holiday destinations from BRS and CWL and you'll see the underlying picture of what is going on.
Fix the whole South Wales economy and CWL will grow in leaps and bounds.