If you do the math, air-launch does not save much in fuel costs, while it adds a great deal of complexity to the launch system, and you do not save much in terms of reduced energy requirements. Most of the rocket fuel and oxidizer are used to increase velocity; comparatively little is used to gain (the first 30k') altitude or lost to atmospheric drag. The main benefit to air-launch is reduced range safety costs, and ease of scheduling, as the launch vehicle can be taken out over an unoccupied stretch of ocean, and range rental/lease costs are basically eliminated.