I don't know. I'd assume they are only sold in countries where it's not common to have an oven. In the USA I've never seen a house or apartment that didn't have an oven included so if my experience matches reality than at least in the USA it doesn't seem like there would be any market.

What other countries is it not common to have an oven?

One top of that there is the issue that if it's not common to have an oven then it's also not common to cook things that would require an oven. So, I have no idea how common these ovens are in Japan in general. Given so many models by so many companies and given they can also be used as a microwave I'd kind of assume they are gaining in popularity. Foods that require an oven are also slowly getting more popular over time but I suspect it's still not common.