There were all sorts of behind-the-scenes factors at work:
At the time of closure the Bath trams had been bought up and owned by the Bristol bus company. Tram fares were fixed by the council, but bus fares were not; the bus company was able to set higher fares if they replaced trams by buses.
It suited the bus company to standardize its equipment by running its buses on the tram routes.
The tramway company had to maintain a lot of the road surface, by act of parliament at their own expense, buses did not.
Because of the enforced low fares, and the fact that the law enabled the council to take over the tram at any time which discouraged investment, the tramway company didn’t have a track and equipment replacement programme in place, so by 1939 a lot of the track was badly in need of replacement.