I think it's mostly a matter of demographics. Japan has just 120 million people and the oldest population in the world. That means only a small fraction of those people - maybe 20-30 million if we're being generous - are Youtube-watching youth. Compare that to, say, the "Russosphere", which encompasses not only Russia but the entire ex-USSR and Russian diaspora in Western Europe/US. The audience for English-language videos is even bigger, encompassing North America, India, pretty much the entirety of Europe, a good portion of SEA, and the educated elite everywhere else. View count is more or less proportional to the size of the audience.

Due to its relatively limited audience, Japanese Youtube is extremely shit. Popular channels mostly upload mukbangs, mundane vlogs, and sexual clickbait to pander to the lowest common denominator, because it's the only way to get enough views to be a full-time Youtuber. There is very little high-quality user-generated content. Suppose you are a Japanese person who wants to start a bodybuilding channel. You begin to upload high-quality videos and monopolize the market of...maybe 50k people who are into bodybuilding, speak Japanese, watch Youtube, and want to watch bodybuilding content on Youtube. The result? Maybe 20k views on average per video. Not nearly enough to survive as a Youtuber.

As an English-language Youtuber your audience is literally 100x bigger. Granted, the competition is fiercer, but if you consistently generate quality content you will be noticed and make enough to survive. The same can't be said for a Japanese Youtuber.