Because it's the evidence of your name, date of birth and nationality, and Japanese offices record those in Japanese. When a marriage with a Japanese national is reported to the embassy or a municipal office, the non-Japanese partner's passport usually needs a Japanese translation.
A Japanese family register (戸籍) doesn't hold a page for a foreign spouse, but it does record your name and nationality on the Japanese partner's entry. The office takes those details from your passport, so it wants a Japanese rendering it can rely on. The Embassy of Japan in the UK, for example, lists a Japanese translation of the foreign spouse's passport among the documents for reporting a marriage.
The UK has no national ID card, so a British passport ends up proving identity, nationality and, in effect, your existence to the ward office, which is a lot to ask of one page.