Public donations made Mutsu hard to sacrifice

Why did Japan object so strongly to sacrificing Mutsu during naval treaty negotiations?

Because public donations had helped pay for the ship, making its surrender politically difficult.

Mutsu was more than another hull in treaty tonnage calculations. Members of the Japanese public had donated toward the ship, giving them a direct stake in its survival. Scrapping it would therefore look like discarding both a modern battleship and a popular national contribution. Japan's treaty position reflected domestic legitimacy as well as fleet arithmetic.