Southwest's reservation and ticketing system is currently not capable of handling the complexities of international travel. The cost of redoing the system is very large as most of it was written years ago and like most old systems is a patch work of poorly documented additions and modifications. It is a major limitation to growth and they expect it to be several years work to modify it.
In the meantime, to continue the growth management thinks is so necessary to their business model, codeshares are an answer.