[bigiain]

I think technology differences play a factor.

CDMA gets longer range than 3/4G, so it's useable in a sparse network so long as it's not trying to service too many people at the same time. I'm guessing the US launched lots of CDMA infrastructure back when it wasn't considered obsolete by most of the world, and hasn't upgraded it's rural cellular networks.

Australia set down all it's "legacy" CDMA stuff over a decade ago, which makes the large dark patches that cover lots of the continent very under-served.