According to the Wikipedia page for eszett [0] it evolved from "sz", as the name "eszett" suggests. (I only realized the link with "z" when I saw "tz" ligatures on street signs in Berlin.) Given that its typographic origin is sz, and given that its name literally says sz, I wish the spelling reformists had gone for sz rather than ss!

[0] https://en.wikipedia.org/wiki/%C3%9F