Desmostylians are found along the pacific rim, and a lot of material is found in Japan itself, with the type genus Desmostylus itself having a Japanese species named. Japan has a much smaller roster of animals found in their border, so the most notable species found there like these desmostylians would be normal to popularize. I'm sure the Triceratops wouldn't be as popular in the US if it was found somewhere like Brazil instead, so in a way Japanese love for the species is in a way a sort of patriotism for their nation.
They were in Russia too but idk the popularity of it there. The US has so much stuff, I can see why these kinda slip through the cracks. Smilodon, Titanis, Megalodon, and other cenozoic wonders kinda take the popularity.