Historical gap in high aspect ratio wing transport aircraft
High aspect wings provide the best efficiency, but the current airline business model needs more than just efficiency; it also needs to keep speeds up.

Modern airlines could save on fuel by flying slower, but they would lose some business to other forms of transport or airlines that did not slow down as much. As long as fuel prices remain under a certain limit, this trend will continue. In this answer, Peter Kämpf speculates that limit might be around 200USD per barrel (originally gallon, but likely a typo).