Throughout the world, when you have two adjacent regions each with their own airport, airlines will always base their aircraft in the region with larger population and higher incomes... and make the poor neighbours travel further. Airlines would be crazy to act otherwise.
Cardiff airport and its traffic reflects the local economy, much like Leeds airport. If you want to see a busy CWL... then you need a really buoyant and dynamic economy in south Wales. You might be able to persuade the odd airline (eg Qatar) to try you out... but in the long term, you need a robust local economy. Anything else is just trying to make a silk purse out of a pig's ear. When you walk around places like Barry and see people struggling to make ends meet, it's not encouraging
Dublin airport used to be sleepy. In the last 30 years, the economy around Dublin has gained vast wealth and the airport is booming. Same with Cork. Yes, the airport is an important asset, just like good roads and a train station... but the way to revive CWL is by creating a strong economy first. Good education, strong skill base, etc...