The 757/767 are comparatively old designs now and perhaps Boeing felt the necessary R&D effort would be better deployed in the 787/747/737 series which had better prospects of longevity, sales etc.
Aftermarket winglets may appear, but the reality is the 757 has ceased production so that model is now entering the twilight of its life, and the 767 is likewise now coming into the latter stages of its lifetime. Operators must weigh up the cost of retro-fitting them over the anticpated remaining life of the airframes, versus disposal of the old hulls and re-equipping with the next generation of fuel-efficient and up-to-date aerodynamics.
Looking good with winglets isn't quite enough to justify fitting them onto older hulls, unfortunately. There have to be some pretty good economic benefits as well, especially in the short to medium term. You want to get a return on the investment, rather than the operator to whom you sell your older aircraft getting it for you.
Last edited by criticalmass; 3rd May 2008 at 10:19.
Reason: spelling