i dont know how a wing is designed powerful, but besides that, the 777 wing was designed in a way that the wingtip vortex is minimal, therefore the weight of any special structure on the wingtip (rake or winglet or whatever) would outweight the gain from it.
that calculation seems to have changed in favour of the raked tip with the 300ER and 200LR, therefore they have raked tips.
it is all a matter of efficiency, similar with the 737NG. to some operators the savings make sense, to others the efficiency gain does not outweight the penalty from the weight, therefore they are not economical.
the wingtip game is just like any other game in aviation, you only add weight if you get enough improvement out of it to make up for it.
in some cases this is just not the case.