A couple reasons:

Soups are often made with leftovers and vegetable trimmings. It’s not that these items are bad—its just very difficult to utilize them since they are odd cuts and sizes. Perfect for pureeing into a soup or just dicing them into a brothy soup.

Why is there only one soup per day? Because the restaurant wants it to sell out. Hypothetically if you offered two soups, the sales of each soup would be cut in half and maybe leave the restaurant with two leftover soups instead of zero.