Fuel availability: Diesel fuel is the same worldwide and available everywhere... it's also cheaper than gas.
Worldwide heating: Heating while driving is allowed worldwide. By supplying the heater with diesel out of the vehicle fuel tank, there is no need to search for bottle connectors or to sort out exchanging gas bottles in foreign countries.
Take up less space in your campervan: Gas bottles are bulky & don't last very long so you normally end up carrying a spare. If you use diesel heating then fuel is simply drawn from your vehicle tank. The heater unit itself is smaller than the gas alternatives and can even be fitted underneath your van to free up yet more space.
Reliability: diesel heater just work and are a lot less fickle than gas devices. Also the fuel wont stop working when it gets colds like Butane is inclined to do in winter.