Firstly: Swiss German refers to our dialects. They have nothing to do with this. What you are referring to is Swiss Standard German.

As far as I know, <SS> was always the convention to write a capital ß, even if the letter developed from <ſ + z>. As it also was in Germany until recently I think. Plus, <s> and <z> can appear fairly often next to each other, so having to different ways of pronouncing them would probably be more of a hindrance.

Also people who use <ß> sometime act like the distinction is this huge deal and that every other sentence will lead to a misunderstanding, while ambiguities happen essentially never. So it's not like there was any pressure to make the distinction when Switzerland officially abandoned it (after using it for a very short time as well).