Brother, can you spare a Mandarin dialect?

Enough people sent me The New York Times story about the changing face - or shift from Cantonese to Mandarin - in New York Chinatown and other places in the United States with large ethnic Chinese populations.
So, I thought I’d use English to join the conversation.
My uncle, Bill Wong, provided thoughtful analysis based on his experience of growing up in Oakland Chinatown – where historically Cantonese (Guangzhouhua) or the Taishan dialect (Taishanhua) can be heard.
Interestingly, given what I’ve learned while studying at Chinese universities, I’m using the Mandarin pinyin spellings for the Cantonese and the Taishan dialects.
Really, given my ancestral ties to Guangdong province, I should be using different spellings. Something similar to: Kwangtung or Toisan.
So, why the rise of Mandarin in places in the world - which largely had Cantonese because immigrants from Guangdong province were some of the first to leave?