Dear Tom,
as far as I know, there are both technical and law aspects that put them down.
Technical: Heavier and slower compared to an IC-engine lorry, frequent need to top up water and coal, more daily and frequent maintenance work required, much more time needed to start up in the morning. More dirt and pollution for both driver and the surroundings. A two man crew was needed, one driver and one fireman like on a steam loco (although the last versions were able to be one-man operated). They were very robust and reliable, but couldnāt compete with the fast progress of the IC-engines.
Law-aspects: The lobby of IC-engine and car manufacturers was also strong in the UK, so they forced the government to enforce higher taxes for steam vehicles. And other rules that favour IC-engine trucks. So they became too expensive to operate.
Steam was also regarded as somewhat outdated, especially on the road.
Compared to an IC-engine with gasification, a wood-gasser is more economical with the fuel.
In Germany, they built some prototypes with a more modern boiler design in the late 30s that were fired automatically with tar-oil from coke production. So they avoided the imported petrol and diesel and a fireman. The prototypes were quite successful, but the the tar oil became more expensive and thus the steam power too costly again.
