That and the fact that Singaporean students are required to study their mother language in addition to English.
Also—aren't most Chinese-Singaporeans of Hokkien or Cantonese ancestry? Their forbears likely didn't speak Mandarin. For that matter, Hokkien was the dominant Chinese dialect in Singapore until relatively recently.
It does, when you consider that there are real communities of diverse groups, each of whom have restaurants. Just last visit (one week, about a week ago) I personally ate at authentic Tamil muslim, Sikh (temple), Karnataka, Burmese, Korean, Lebanese, Yemeni, Italian, French, Sichuan, Thai Muslim, Thai and Thai vegetarian restaurants.
That doesn't touch on Nepali, Yunnanese, Khmer, Vietnamese, Malay, Japanese, Indonesian, Philippino, African, or other Chinese or western cuisine. For sure there are many more Middle Eastern places too, Russian and eastern European, plenty of Australian, US and Canadian places, as well as South American, etc. I've eaten Iranian before there too.
Nor does it touch on the significant local cuisine of the city (some of which is Chinese-derived), or specific regional cuisine from other parts of Thailand, virtually all of which can be found in Bangkok and virtually none of which can be found intact in Singapore.
Singapore is actually unusual compared to other diaspora communities in that most of the emigrants were from Fujian province, not Canton.
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.
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.