Note that even SQ doesn't fly non-stop to SIN from LAX or SFO, opting for SFO-NRT-SIN, SFO-HKG-SIN and LAX-NRT-SIN direct routings.
I think that's more a factor of having the right aircraft. SQ has already stated they plan on re-starting non-stop flights to the US with the A350-900ULR