Probably for the same reason that Swiss trains around Zurich have German and English announcements (sometimes additionally French), and around Geneva have French and English (sometimes German), but I have yet to hear Italian announcements on a Swiss train as I have not been to Ticino.
In the Italian area they DO make announcements in Italian (and sometimes English and/or German).
On trains in the Graubeden area in Romansch and English and/or German--occasionally English
only.
Switz. is very tourist oriented, so using "other" languages are based on the "audience" they are dealing with. In Zermat there are lots of signs in Japanese--I've arrived in Zermat and the station plaza was wall to wall Japanese (tour groups).