..another important concern with the big birds is the existing airport apron infrastructure.... I have no doubt that Mr Boeing would have far preferred to go for an increased wingspan .. but the bird then wouldn't fit into the gates without a major (read super expensive worldwide) rework ... so the only effective way to get a reduction in induced drag is to go for something more brutal ....
I suspect that, for many of the smaller aircraft, marketing has had a say in the way drag reduction is approached .....