From what I have read in the past, the .357 Max brass if formed from .223 brass in one of the final stages of drawing. Maybe, they are making so many .223's and 5.56's that they don't profit as well with the .357 Max. If I can find my print outs concerning the .357 Max forming, I will try to post a link.
I believe if Remington drops the .357 Max. the good folks at Starline will be glad to supply our needs. They produce .357 Mag and .360 Dan Wesson brass so it shouldn't be too hard to get set up and running.