I think it has something to do with increasing the Aspect Ratio. Winglets create a smaller increase in the Aspect Ratio, and the associated decrease in induced drag is thus less.
Raked wingtips are a quick way to increase wing area, without making a massive modification to the wing structure (stringers and spars mainly). They can be designed to distribute the loads so that they have minimal impact on the outboard wing section.
Something like that anyways!!
