Another point - whilst a fairly large number of the Welsh catchment uses Bristol, remember that they are spread across a large number of destinations. If they were all on say the Rome, then yes, a Rome service from Cardiff may make sense. But when it is a question of ten on this flight, fifteen on that one etc etc, then none of them make sense from Cardiff.
The argument about an airline being willing to 'try' is a little silly - a whole load of research goes into a new route decision, and it's most certainly not a case of 'suck it and see'. Now and again, they get it wrong - Planenut's comment about DSA proves that - but most of the time, they get it right. Airlines, especially in this economy, are very risk averse - and Cardiff is seen as a risk, based on history.
Xtypeman's last comment should be stickied at the top of this thread. Cardiff's market is relatively small and relatively poor, and airlines would rather go someplace else.