Further to pipertommy's comments, here is my opinion
I think the main problem is that we shouldn't compare Bristol with Cardiff anymore. Both airports are in different leagues. Like it or not Bristol does have the edge.
One of the main reasons for this is that Cardiff's catchment area comes of roughly 2 million people. Bristol on the other hand has a catchement area of roughly 4 million people, the majority of whom have an average GDP much higher than the average in Wales. This means that the Bristol market is not only larger in size, but more productive in terms of higher prosperity of that population to fly.
The economics speaks for itself. The higher the catchement area the more choice/services you're gonna get