In Japan, space is really rare, and with the average home in Tokyo being only 66 meters squared (compared to 164 meters squared in New York), people don’t have the luxury of owning so many possessions.
Home size certainly plays an important role in the non-materialistic nature of Japanese people, but it is not the only reason some people live simply. Zen Buddhism also preaches simplicity, encouraging people to free up their space at home so that it may free up their mind. Of course, Tokyo is the number one location with the most minimalists, so space definitely is the main reason.
But there aren’t just these 2 limiting reasons for not having stuff; there are practical ones too. Owning fewer possessions is a lot cheaper. If you don’t buy stuff, you will have more money. It’s also easier to clean a house with barely anything in it than one that is completely cluttered. And finally, Japan is a country heavily affected by earthquakes, which could easily knock things over if they are strong enough. Half of reported injuries during earthquakes occur from objects falling on victims. Living life simply in these cases is definitely living life safely.
For some people, owning possessions brings joy to them, as does the rush of collecting more and more things. But for minimalists, they can’t see any good out of such behavior. Owning simple things, and the bare minimum, helps them live a stress-free life where their bodies are free to stretch out in the open space, and their minds are free from distraction of material possessions.