The potential viewership of content, whether it be movies, television shows, or videos, is greatly impacted by the addition of language subtitles, giving it a competitive advantage in terms of worldwide reach.
If the viewership was restricted to a small population due to the lack of language subtitles in the content, it would be a pointless exercise given the time and work that goes into developing any content.
It would also be foolish if English-language content lacked Mandarin subtitles, given that the majority of people in Taiwan, Singapore, Macau, and Hong Kong, in addition to China, where Mandarin's roots lie, speak the language.