Visbility of both them and the alternatives (and perhaps the people and uses). Compare that to say bunker fuel in the cargo ships. We don't see the ships, we do

Visbility of both them and the alternatives (and perhaps the people and uses). Compare that to say bunker fuel in the cargo ships. We don't see the ships, we don't understand the fuel, I would struggle to name an alternative propulsion better than something in early research like those spinning 'sails'.