Why do the descendants of immigrants from Shanghai, Chaozhou and other places in Hong Kong speak Cantonese instead of the language of their ancestral home?

The descendants of immigrants will be assimilated by the local language. Because Cantonese is widely used in Hong Kong, descendants of immigrants from all over the world speak Cantonese. Unless their parents speak their ancestors' dialects at home, most of these descendants can't speak their ancestors' dialects. Similarly, many China people born and raised in the United States and Canada can't speak Chinese.