Other people have mentioned the new, higher-efficiency planes, but the reason they've had this much effect is that they've caused a fundamental shift in the arc

Other people have mentioned the new, higher-efficiency planes, but the reason they've had this much effect is that they've caused a fundamental shift in the architecture of airline routes by killing the "hub and spokes" model. Instead of using small planes to move passengers to the big hub airports, it is now faster and cheaper to fly direct even for long routes, turning decades-old wisdom about airlines on it's head.