My thinking is they are significantly cheaper to buy and fly, and presumably much safer for pedestrians too. Maybe they could also form a mesh network to avoid a single point of failure.
If you want a drone that's safer than the average helicopter, you're going to be paying a lot of money.
* about more professional drones
If you're flying a drone in proximity to the peloton, it's not a question of if you'll lose your control signal link, but how often. I don't know how anyone could operate a drone in that environment with any degree of confidence, even if it's capable of GPS position hold. What happens if you lose your control link just as your battery is running flat? What happens if your chase vehicle gets stuck in traffic and your drone is stuck loitering on the other side of a tunnel? How can a large, heavy drone autonomously find a safe landing place on a mountain stage that's littered with thousands of spectators? A drone big enough to carry the requisite broadcast equipment is undoubtedly heavy enough to kill a bystander.
Broadcasters and race organisers have been using helicopters for decades and know how to manage them safely. Drones simply present too many unknowns. To the best of my knowledge, no helicopter has ever crashed while covering a cycling race. There have been a few incidents where the downdraft from a helicopter caused riders to crash, but none of them were serious.
I think you can do the math from there.