Lower labour costs, and no unions, maybe? D'Addario ( $AUD 15 to 20 ) have a Darco range which is assembled in Mexico as well, sell for around nine bucks a set here. I didn't use anything special or change strings more than every five or six months until one day I backed a friend on his CD and heard the first run we did from months back- wow, the lack of resonance and attack after the strings were older changed my mind fast, and so did the DR's and Dr Tomastic Plectrum strings he was using on his gear...they left the Fender's I had on for dead.
Really, if you can improve your sound so much- and the increased speed and lack of finger-scrape if you use coated strings- for the sake of 5 or 10 bucks every three months or so I think it's well worth it.
I suggest people change their strings every four months if they are a basic student, and three or less months when they are more advanced or are playing more.
Elixers, John, are not a cheap string..they are 35 bucks a set here when a good set is $20 and a cheap but OK set like the Darco's is half that, but I'm convinced that if you were to try DR's, or Plectrum, or best of all Elixers, you wil notice a remarkable lift in brightness and tonal response, and speed as well.