Why is it that UA's flight boking website never offers me SQ1 leaving SFO for SIN (as a return) at 1:15am, not even as a 'partner flight'?

I can even specifically select '1 am' departures and it is not shown. Fare types also don't seem to make a difference.


(In general though, the selection of the departure time in teh flight search seems to be generally useless in most cases anyway).

Is there some difference in partnership between UA and SQ over UA and other star partners?