USA RVSM
Piniped I think the answer to your question is that the US has not only more aeroplanes than anywhere else but also more old aeroplanes than anywhere else, particularly high flying freighters, biz jets and the like. The cost of fitting them all for RVSM is astronomical and the operators are still fighting the move. It will of course come because it dramatically increases the efficiency of the airspace ---but back to the real topic