This letter stood for /ʃ/ in Aramaic. 𐢖 was not adopted into the Arabic alphabet because of the following considerations:
Proto-Semitic has three fricatives that became sibilants in descendant languages: ś, š, s. In Arabic ś gave /ʃ/, but š and s merged to /s/. In Old Aramaic all three sibilants were distinguished, and s was represented by 𐢖 / ס / 𐡎 while 𐢝 / ש / 𐡔 represented both ś and š. This orthography persisted in later Aramaic even to the time of the Nabataeans, when the pronunciation of ś merged with that of s/s/.
So when a scribe wanted to write the Arabic sound /ʃ/ from ś, he did so by using 𐢝 / س purposefully in order to stick to a letter form that looks the same in the cognate Aramaic words. But when he wanted to write an Arabic sound /s/ from Proto-Semitic š, he would defy the Aramaic spelling for this Proto-Semitic phoneme were he to represent this Arabic phoneme by the sign 𐢖 / ס / 𐡎, whereas Arabic-Aramaic word pairs with the Proto-Semitic s are – somewhat by coincidence – rare and there was hence only negligible pressure to employ 𐢖 / ܣ / ס / 𐡎, in total. So the impression was that 𐢝 / ש / 𐡔 / س is the most typical sign to write /s/ while the other possible sign fell out of use.
Diem, Werner, “Untersuchungen zur frühen Geschichte der arabischen Orthographie: II. Die Schreibung der Konsonanten”, in Orientalia (in German), volume 49, issue 1, 1980, DOI:10.2307/43075525, pages 77–81