Pehaps it is Price vs benefit.

At UAL, they told us the maintenace cost on the 747-400's winglets almost negated the fuel savings...and this was in 2000/2001.

What maintenance costs? Well, there are a lot of bending loads on these things, and they tended to cause cracks in the structure at the wingtip. They spent a lot of time and money fixing them--sometimes planes were out of service as a result. THAT'S expensive.

Most STC mods I have ever heard of are hideously expensive. I'm sure the winglets are in the multi-million dollar category for a 757. You'd have to save a lot of fuel to justify them (and they only save about 1.5-2.0%, according to reports I have read).