What do you think is contributing to more cargo being flown international versus domestic to cause different revenue ton miles (RTM) ranges?

I'm sure there are many factors that contribute to this phenomenon, but I would expect an emphasis on international trade has to be one of the current reasons behind it. As the economy strengthens and improves internal manufacturing (instead of outsourcing its manufacturing and production to other nations), the exporting of goods to foreign countries increases. This means that more goods are going to be transported via airfreight or via boat instead of railroad.
Additionally, the decreasing cost of fuel and increased safety and capacity of air cargo has certainly improved the amount of goods that are able to be shipped internationally using air freight. This, coupled with a marketplace that is more global than ever, will certainly increase the amount of air freight that occurs to other nations.