I think it started as an affordable alternative to stick frame construction in a few instances where the containers were readily available, but I agree it's become something people want to do even when its not cost efficient in their area. On the other hand, why shouldn't building a house be as simple as joining two or three prefabricated, easily transported, container-sized modules together? It just needs the right price point.

[edit] There's a company called Bone Structure that has a modular steel building system that's almost a hybrid between pre-fab and on-site custom. Still expensive though.