It almost certainly has more to do with your society's overall approach to sickness than it does with just masks specifically. Here in NYC, loads of sick people have no qualms with squeezing in between two people on the subway, coughing into the same hand they're about to use to turn a doorknob with, and so on. Wearing masks might help a tiny bit against inconsiderate people like that, but not a lot. I'm sure such inconsiderate people exist in Hong Kong too, but I'd be willing to bet that there are fewer, and that wearing masks is an indicator that more people are actually considerate about their sicknesses in HK than in the US.