You are forgetting countries such as Russia or Kazakhstan. Some miners surely migrated there as we can see from the increase in hashrate in those territories. It's closer to them than USA is and presents a decent alternative.

Packing up and leaving the country and the continent to go to the US to mine Bitcoin isn't as straightforward as many think. They are people with families and children who live and work in China. It's not that easy to leave all that behind. You need to purchase or rent a location from where you will mine in a brand-new country with unfamiliar laws and language (maybe). The equipment that consists of thousands of miners has to be packed up and shipped overseas. This is also expensive and requires additional paperwork and time.