Why do Bun Foo and Ching Ho from Thoroughly Modern Millie speak Cantonese and sing in Mandarin Chinese?

I was recently casted as Bun Fo in my school’s production of Thoroughly Modern Millie. In the show the two Chinese brothers speak two languages: Cantonese when they are talking and Mandarin Chinese when they are singing. Is there any particular reason why the playright did this?
Thanks for your answers! 🙂

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