They just don't need it
The winglet should be something the enginneers use to fix a problem without having to redesign it all over.
In all cases in commercial aviation, winglets appeared to solve weight problems, so the basic wing design could be preserved, wich saves a lot of money.
examples: MD-11: they couldn't stretch the DC-10's wing enough to suport the increased weight of the MD-11 over the DC-10 (remender, the original DC-10-10 wing had already been stretched to build the DC-10-30)
A340: Airbus was expecting to use new Super Fan engines from IAE, wich did not came to life, so they were stuck with less powerfull CFM engines. To improve performance, probably 2nd segment performance, they added the winglets
B747-400: same thing. New version, more range = more weight + same wing = winglets
ERJ-145XR: new version, longer range = winglets