Swansea does not have the population or the inbound demand to support an airport for scheduled services. The minute you open the door you need a raft of employees to cover fire station, ATC, baggage handling etc etc. This is before you even consider security staff and maintenance costs and very substantial initial investment... Nowadays to make an airport work you'd need circa 1million passengers to make the operation stack up and that presupposes the airlines will pay a going rate for using the facilities, which is not always the case.
Unless ocourse there is a huge government subsidy, which is in my can not be justified. Until recently Cardiff had been struggling and the last thing that is needed is another airport in the same neck of the woods.