Japan is densely populated and small. Tokyo has the same population as the entire state of California. A 'grassroots' campaign or a 'try it local first approach' simply takes into account a staggering amount of people, and all of these people have relatives that live within 3 hours distance of them, due to the size of Japan and the speed of transport.

It means that though Japan is largely conservative... if an idea does take root, it can quickly spread everywhere.

In the US, you get enclaves. California says X. Texas says Y. New York says Z. And there's 1500 miles between each of these major population zones, meaning that activists and politicians in one zone simply aren't active in another.