Because of these factors, some streetcar companies began going into bankruptcy as early as the 1920s, when they were still their cities' dominant mode of transportation. Huge costs and the falling value of fares forced them to cut back on service, steadily pushing people to the convenient, increasingly affordable automobile.Easier to demonize the traction magnates and subject them to regulation. But perhaps the development of personal transit will undo any transportation company. Note that the jitney operators who sold seats in their automobiles had to be regulated as taxi companies, and with smart 'phones, jitney operators don't have to cruise the streetcar routes to find riders.
As they fought to stay alive during the Great Depression, many companies invested in buses, which were cheaper and more flexible. Initially they operated mainly as feeder systems to bring commuters to the end of lines, but as time went on, they began to replace some lines entirely.
That wasn't enough to save most of these companies, especially as city, state, and federal governments pumped more and more money into roads.
To the usual list of reasons, add government complicity in creating the suburbs, and a related Vox essay suggests that transit systems are part of the Welfare State, not necessarily for all citizens in the same way schools or libraries might be.