Japanese writing systems share a large set of logographic characters with Korean and Chinese that are collectively titled CJK characters. There are a lot of these characters and not many designers are willing to create fonts for them compared to Latin fonts. So, Japanese web designers have less fonts to choose from and they can’t use capitalization to form visual hierarchies.
I reckon these limitations not only apply to web, but to any creative output. It is fascinating how language frame the way we communicate and how. Any Japanese around here?