Because Buses and Cars were better than all the infrastructure tied to trolleys. a road provides more options and less capital.
Its also why a lot of freight moved to just in time inventory, no rail maintenance and flexible trucking options
the taxpayer pays for the road and maintenace
You could argue that we should just have buses instead of streetcars but personal cars are clearly inferior in urban environments. They cause enormous pollution, take up tons of space, and transport very few people as compared to the other options.
Cars are really bad to have in dense cities!