The B777 has what is known as a 'super critical wing'. The design is such that the Critical Mach Number (M-Crit is the speed at which supersonic airflow first occurs on the airframe), is higher.
As well as raising M-Crit, the wing is designed to give less drag at low speed.
I guess in the end they figured they had done a good enough job in designing the wing to no longer need winglets.
ODL