It is the typewriter. A Swiss typewriter needs to support three languages: German, French, and Italian. Therefore on the Swiss typewriter, there was no ß key. It also has only lowercase umlauts ä, ö, and ü. A picture of a Swiss typewriter can be seen here.

The lack of that key has led to a subsequent deprectation of the ß overall.

It is because of the typewriter. A Swiss typewriter needs to support three languages: German, French, and Italian. Therefore on the Swiss typewriter, there was no ß key. It also has only lowercase umlauts ä, ö, and ü. A picture of a Swiss typewriter can be seen here.

The lack of that key has led to a subsequent deprecation of the ß overall.