I would guess:

1. Simplicity (less to break, repair)

2. Weight

3. Safety: There would be more instances where they might cause more harm than good.

(for example minor impacts or large jolts might trigger airbags, thereby rendering the car and driver incapacitated... when they would normally still be able to drive)

4. Safety #2: The drivers have already are wearing advanced harnesses and helmets which provide a similar restraint effect as airbag.

5. Summary: They wouldn't really serve a purpose other than getting in the way