The only other things I can think of is A) that this is the 'conventional' or 'traditional' flow of service non-frequent passengers come to expect when flying and so SQ kept it, i.e. board > take-off > meal > sleep / work / IFE / whatever > 2nd meal if there is one > land or B) this is how SQ management want it done

It also comes down to passenger preference, some want lunch and other would like dinner. It's like those LH flights where the 2nd meal is labelled 'Breakfast' because your first meal was 'Dinner' but you're arriving at 9pm destination local time. Some find this fine, others find it weird since they're already trying to acclimate to the new time zone and would have preferred dinner as the 2nd meal.