Max,
Winglets don't _have_ to be more efficient in cruise, but as they are fixed, they are set so that they are creating forward thrust most efficiently under just one flight regime. The manufacturer can choose what flight regime that is. In the flight regime that is not optimized you can actually lose performance.
The RJ obviously has been been optimized for some flight regime, or maybe a bit inbetween (sort of like a fixed pitch prop, it is ok for climb and cruise, but not best for either). Perhaps they figure a small amount in both regimes will net more in that airplane. However, a 777 does either short haul or long haul on any given flight, and you would have to choose which one to optimize. Since the same airframe might alternate from day to day, the other solution is to increase the span with the super-critical wing, as they have done.