Not only the romanization, also the capital form in German is SS.

The ß sticks around in German because it prevents some ambiguities. For example Busse and Buße are different words with different pronunciations.

Basically German orthography backed itself into a corner because of two rules: a vowel before a doubled consonant is short, and an s before a vowel is pronounced /z/. This leaves you with no clear way to write a long vowel followed by an /s/ sound followed by a vowel: VssV would make the first vowel short, and VsV would keep the first vowel long but the consonant would be pronounced as /z/. The ß solves this problem.