It's because the Singapore government years and years ago created the "cooked food centre" approach and implemented it in standardised uniform fashion everywhere: that's the Singapore approach to everything, favoured by the late Prime Minister.

Hong Kong is far more laissez-faire. Mall owners and stall owners are free to organise food centres as they wish, if they organise food centres at all. As you say, free-standing eating places on the street are far more popular in Hong Kong.

So it comes down to a difference in local cultures and government style.

My own thought is that the Singapore cooked food centres now seem sort of dated and '70s-ish, but maybe that's their appeal for some.