EA is now saying offline mode was rejected because it didn't fit their vision.
previously they said it was because it was impossible. EA Says It Rejected Offline Play For SimCity Because “It Didn’t Fit With Our Vision”
[W]e rejected that idea because it didn’t fit with our vision. We did not focus on the “single city in isolation” that we have delivered in past SimCities. We recognize that there are fans – people who love the original SimCity – who want that. But we’re also hearing from thousands of people who are playing across regions, trading, communicating and loving the Always-Connected functionality. The SimCity we delivered captures the magic of its heritage but catches up with ever-improving technology.