I cannot comment upon why the aerodynamacists at Boeing chose not to use winglets on the B767/777, but it's important to appreciate that winglets are intended to alleviate Induced drag, i.e. Low Speed Drag, whilst the Super Critical wing is designed to alleviate High Speed Drag, i.e. Wave Drag due to trans-sonic shock wave formation.
One has nothing to do with the other.
Regards,
Old Smokey