Cough pretty much nailed it. Boeing believes the raked wingtips to be a better solution than winglets, wingspan permitting. Hence the folding wingtips on the 777X. For the commercial 737, adding 10+ feet to the wingspan would create problems fitting into the gates, therefor winglets are used.
The winglets require quite a bit of structural strengthening of the wing - a buddy who worked the 757/767 winglets for Aviation Partners said it cost ~$1 million to install the wing structure that allowed the winglets to be added - it's not just a bolt on addition to add winglets.