Mandarin is a foreign language, brought in by a foreign colonial government 70 years ago, and forced on the people with extremely heavy punishment (social and economic, if not corporal)
Taiwanese was spoken by the large majority of Formosans prior to that. Probably 75% natively and most of the remainder as a second language. It even did fairly well under Japanese rule, relatively speaking.
Most importantly, Mandarin brought with it an entirely foreign underlying culture around “Hanness” and Tionghoa (“Chunghwa”) - the things we confusingly label “Chinese” in English - which was not widespread in Taiwan prior to the ROC arrival, and is the main reason for the ongoing PRC-ROC conflict.
At the very least, general Italian (Tuscanese?) is an Italian language, and the cultures are at least similar or compatible. I don’t know if there were harsh punishments for using a different language at points in Italian history, but in Taiwan that situation lasted for a full two generations. And I don’t think France is ready to go to war with Italy if they switch from Italian to Neapolitan.
FWIW when I did a semester abroad in Italy, it seemed fairly common for Neapolitan, Sardinian or other students to speak their own languages when they were together.
(Not to say other Italian languages aren’t at risk or anything - I’m sure they are - but the situation is not comparable to Taiwanese and Mandarin, not even remotely.)