The issue is somewhat religious to many people but I think this side of it makes sense, although I haven't done the calculus myself.
Supposedly, when structure and available room (airport size that is) gives you the option, you always benefit more from adding additional wing span than the same weight of winglets. When designing a new plane as opposed to retrofitting an old one, you do have this option.
Sounds like a plausible explanation to me.
Cheers,
/ft