The problem with narrowbodies at HKG has to do with slots and cost of the main gates. It's just not practical for CX to have a massive fleet of narrowbodies, at least until there are more gates or a new runway.

The airport charges quite a bit more per hour for the main gate. Not all gates and services cost the same, and the gates CX use are in the highest category.

And there aren't too many more slots to go around unfortunately, either. It's not like CX or KA can increase that much more frequencies even if they want. If they want to keep launching new destinations, they can't really say double the number of TPE flights by adding a bunch of A320s or B737s. They're using all their additional slots on new routes.