Several reasons: - autonomy is in the 1/2h range for rotary-wing drones, you would need a lot of drones/operators throughout the race (it lasts 6h on average). - the cameras/lenses used are huge, in the tens of kg, you could be closer with smaller drones thus requiring lighter lenses, but still, I think you would need big drones to keep the same quality. That means less safe and cheap. - speed, the riders can reach 80-90kmph when going down, which is about/higher than the top speed of most drones. - worse visibility/reactivity than a team in a helicopter, so harder to adapt to the action. - better weather resistance

All around, helicopters are still the better option right now. I guess they could/do use drones for some specific sections: climbs with a lot of public and similar where endurance/speed is less an issue and you can go much closer.