The director of All Live probably often has to choose the signal that is working the best. The onboards seem to cut quite often. And I think the onboards are working by an aeroplane flying over the stage, acting as a receiver to the transmitters in the cars. If the plane is too far or the terrain is blocking, the signal does not work. This is probably the reason why the Saturday Ittiri stage wasn't shown on All Live in Sardegna, the plane wasn't there. In F1 this is a lot easier because the cars never go that far from the receivers, whose positions can be optimized and tested beforehand.

Another current sport which allows multi camera viewing is soccer, but again, it's not like you can choose which player to focus on, you have like the main view and then a couple of special angles.