Why mutton soup is a high-end snack in Japan?

The earliest mutton soup was made of mutton and cooled into frozen soup to accompany meals. Later, it spread to Japan with Zen. Because monks don't eat meat, they steam red beans mixed with flour or Ge Fen. So mutton soup has gradually evolved into a jelly-like food made of beans in Japan. Later, with the development of tea ceremony, mutton soup gradually became a famous refreshment.