As a fan of streetcars, I have to say there is a lot of general missunderstanding about the systems and how they worked, and why they went bust. Streetcars were in fact much cheaper to run than buses in a simple sense of cost per passenger - but building and maintaining the infrastructure was incredibly expensive. Aside from being required to maintain roads they ran on, most companies also operated their own power stations (many systems predated widespread adoption of electric lighting in homes and citywide power grids) which meant even more infrastructure to maintain.
Aside from being unable to raise fares whenever needed (it's not true that fares were never raised, but often when requesting to raise fares cities would demand a "compromise" resulting in a higher fare that wasn't yet high enough - I believe Fort Collins was the only system still operating on a nickle fare by the 1940s) several other factors played into the streetcar's demise in the U.S. The biggest issue, which is hardly ever brought up is that during WWII streetcar ridership soared to levels that hadn't been seen since the 1920s. But since materials were rationed, it was impossible to properly maintain these systems. When the war ended companies were faced with the option to basically rebuild the systems entirely and replace all the rolling stock - or scrap everything and buy some buses. The former option was often the only practical way to stay in business.
The other thing that helped killed off streetcars, unsurprisingly is cars themselves, or rather the traffic they created. Denver was one of few systems that kept streetcars in their fleet after the war. Although they had already planned to phase them out - gradually - they ended up rapidly ending service because of the city's plan to adopt one way streets to deal with traffic. To complete a gradual phase out as planned would have required tearing up lines and relaying them on streets where they wouldn't run against the flow of traffic. So instead they just tore everything out as quick as possible and forgot about it. This particular scenario played out in many other cities too.
The final straw was also just that of perception. Streetcars were seen as old fashioned, and many people just didn't want to see them on the streets.