what i have heard is that of revenue protection.
When the non-stop service was introduced, it was at a time when most airlines were trying to do away with F class if possible. On some of the local dailies in singapore, there was an article that 1 F seat actually occupies 2 Raflles class seats but the way the article was written, it was clear that for the space that F class is occupying, better revenue could be sought from having a Raffles class configuration. And Business class seats on most airlines tend to be sold easily on most airlines at that point in time.

But that doesnt mean SQ doesn't want to keep F. They do but they only use it on extremely full routes like the LON-SIN and SYD-SIN routes. On some routes like CHC-SIN, they use the 777s that do not have F class.

To answer the op question, true that nobody knows the real answer unless you are with SQ marketing but im pretty sure it has to do with revenue protection and the weight restriction.