Probably due to its close proximity to Japan. There are many Japanese restaurants and Sushi bars in Hong Kong. Just in my residential area alone, there is at least one Japanese restaurant at every corner of the street.
That makes loads of it in each living area. The Japanese restaurants in Hong Kong are generally good and there are many around with good standards that are value for money. I would say its easier to find good quality Japanese food that are more affordable than in Singapore.
Just sidetrack a little, I was looking through the local papers and noticed that Japanese speakers or translators are paid lesser wage than those in Singapore. I think that might be because there are more Japanese speakers in Hong Kong.