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It's due to the fact that Cantonese is the main language that Westerners will encounter if they ever visit their local Chinatown.

Singapore is actually unusual compared to other diaspora communities in that most of the emigrants were from Fujian province, not Canton.

[abjwam]

Don't think that's true. I'm in southeast asia and the largest chinese language groups in the following countries are:

Singapore: Hokkien

Indonesia: Teochew

Philippines: Hokkien

Malaysia: Hokkien (although cantonese popular there)

Thailand: Teochew

Teochew and Hokkien are actually part of the min-nan dialect group which also includes Taiwanese.

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That's fair, but anything outside of SE Asia is predominantly Cantonese, hence why Westerners are familiar with it.

There's also the fact that the HK film industry was fairly dominant for a couple of decades--if a Westerner saw a Chinese-language film before the 90s, it almost certainly had Cantonese in it.