Just to be clear, I'm talking about a Japan that becomes communist from within and prior to WW2, not one that gets Operation Downfall'd into communism. Whenever it gets brought up, it's dismissed, but I don't see why it's so fundamentally unlikely.
It can't be the lack of development of an urban proletariat, since urban workers were (IIRC) less than 20% of Russia's population pre-1917. Japan was also developing on the back of feudal structure, just like Europe, so its class system had similar roots to that of Europe (and Europe was quite susceptible to socialism).
Militarism doesn't seem to make a whole lot of sense either. Plenty of Latin American nations have oscillated between military rule and socialism, and Russia was most certainly a quite militaristic nation, internationally speaking. Is it the fact that the militarist system in Japan was so successful, with all their military victories up until the Second Sino-Japanese War, so the populace wouldn't be liable to start hating the military structure that's done so much for them? It seems like it'd be pretty easy to have a PoD that's Japan loses the Russo-Japanese War or something, disgracing their military structure.
The whole "democracy is the anathema of radicalism" doesn't make a ton of sense either, since democracy developed pretty late in Japan (during the Taisho era).
I'm honestly having a good deal of difficulty in trying to come up with more reasons why Japan couldn't become communist. So why is it assumed that Japan couldn't?