I'm honestly surprised Hepburn wasn't the official standard yet. It sounds way closer to the spoken sounds, at least to my western ears.
> The council’s recommendation also adopts Hepburn spellings for し, じ and つ as shi, ji, and tsu, compared to the Kunrei spellings of si, zi and tu.
I could imagine si, zi and tu sound closer to the spoken sounds to Mandarin speakers.
One issue holding back the adoption of Hepburn has been that the standard national curriculum (gakushū shidō yōryō) calls for all children to be taught romaji beginning in the third grade (previously fourth grade) of elementary school. It's taught in Kokugo (national language, i.e., Japanese) classes and included in those textbooks, as romaji characters are used in Japanese alongside kana and kanji as well as, increasingly, in daily life (user names, passwords, etc.). At that age, native speakers of Japanese can acquire kunreishiki more easily, as the consonant representation corresponds more closely to the Japanese phonology that they have internalized.
For pinyin representation of Mandarin, these are very different sounds, while the equivalent (identical) Mandarin pinyin representation of し, じ, つ would be xi, ji, cu. I'm not as familiar with romanization systems closer to Latin pronunciations, but for Wade Giles it would probably be written like shi, chi, tsu.
Not closer to the spoken sounds, closer to English orthography.
The popularity of Hepburn has a lot more to do with the English language than the Japanese language
You mean, if you would apply the inverse of the standard romanization of Mandarin, the resulting sound would be closer to the Japanese sound, if starting from the Kunrei spelling than if starting from the Hepburn spelling?
> It sounds way closer to the spoken sounds, at least to my western ears.
That's the thing... to some other non-English language speakers, the existing/old romanization method actually is more accurate regarding how the letters would be pronounced to them, especially coming from languages that don't have the same e.g. [ch] or [ts] sounds as written with Hepburn.
The one technical downside I would say to this change is, 1:1 machine transliteration is no longer possible with Hepburn.
I don't know the details history of the system's development, however I notice that with Kunrei everything spelling is neatly 2 characters while with Hepburn it may be 2 or 3 characters:
Kunrei: ki si ti ni hi mi
Hepburn: ki shi chi ni hi mi
The politics of the issue is obviously that Hepburn is older and an American system while Nihon and Kunrei are very purposely domestic (Nihon "is much more regular than Hepburn romanization, and unlike Hepburn's system, it makes no effort to make itself easier to pronounce for English-speakers" [1]). Apparently, Hepburn was later imposed by US occupying forces in 1945.
Perhaps 80 years is long enough and suitable to effect the change officially with no loss of face.
The old official system arguably makes more sense from a Japanese perspective.
If you look at the kana, the Japanese syllabic writing system, they have this ordering: ka ki ku ke ko, sa shi su se so, ta chi tsu te to, etc. If you follow the regularity where there should be a "ti" sound there is no "ti" sound and it happens to be pronounced "chi".
One common analysis holds that the underlying phonemes really are: ta ti tu te to. Traditional Japanese grammarians usually analyzed it this way. And they were historically pronounced that way: it has arisen out of relatively recent sound change. Somewhat like how some British speakers pronounce "Tuesday" such that it sounds much like "Chews-day" to speakers of other dialects. Affrication in a fixed context. The t phoneme triggers that kind of affrication obligatorily in Japanese, before the i vowel or y glide.
Some disagree with this as overly theoretic and based excessively on historical linguistics, and they insist that sh and f and ch are distinct phonemes in Japanese. But the Japanese writing system itself treats it as if they were not.
If you are learning Japanese it makes sense to pick a system that reflects the internal logic of kana spelling. If you want to just approximately pronounce Japanese words in English then you want something that reflects the logic of English spelling.
These two goals are always in tension. Mandarin pinyin, for example, was designed to reflect the logic of Mandarin phonology in a consistent way. It's not meant to be easily pronounceable by English speakers. It's to enable Mandarin speakers to look up words in a dictionary or for students of the language to study Mandarin. Though it has ended up used as a pronunciation guide for English speakers. And that often doesn't go well; a lot of English speakers don't know what to do with the q's and x's.