It is reported that SQ looked at the A332 long and hard as a A310 replacement, but the aircraft was not what they had in mind. Absolutely nothing wrong with the A332, great plane, but SQ simply decided it was not the right plane for them and the routes it would fly.
The 777 was chosen since it was already in the fleet and SQ decided, I assume, that they could use the extra capacity on most routes and that is was more economical than introducing another type into the fleet.
I am not convinced that, after the A310s are retired, that the 777 will be the smallest aircraft in the SQ fleet for long. Much depends on the future of Silk Air, which flies the lower-capacity routes for SQ. Having a 300 pax jumbo as your smallest aircraft is a bit unusual, but, then again, SQ is not your usual airline.