I'm going to illustrate this using the example of Los Angeles, because it's what I know best. (I'm currently writing a book on the subject.)
In general, American streetcar systems reached their height in the 1920s, and began to decline thereafter. This is due to a combination of three factors: (a) the automobile's increasing popularity made many streetcar lines simply uneconomical to operate; (b) most streetcar companies like the Red Cars in Los Angeles or the Key System in the San Francisco Bay Area operated their streetcar systems to promote their real estate developments, and not the other way around, (c) the bus was actually a technological improvement in mixed traffic over the competing streetcar.
If you check out my (redrawn) map of Los Angeles's Pacific Electric streetcar system near its height in 1926, there's lots of things that are incredibly unusual to modern eyes. First, the streetcars go everywhere. You could take a train way out to places which are considered the periphery of Greater LA even in modern times. These are places like San Bernardino, contemporary population 37,000, or Riverside, population 29,000. This kind of network is only economically viable if there are literally no other transportation alternatives available. And the Pacific Electric was infamous for exploiting its transportation monopoly to the limit. Once the motor bus was invented, and automobile competition arrived in earnest, large-scale abandonment began. If you check out this significantly-less-clear map from 1947, nearly all passenger service to outlying areas has been abandoned or replaced by buses.
You have to remember that the Pacific Electric and similar systems were operated by real estate developers. The owner of the Pacific Electric, Henry Huntington, was a real estate magnate who made his money by building suburban subdivisions - places like Huntington Beach, Huntington Park and San Marino were all developed by Huntington money. In the Pacific Electric's case, the streetcar system actually operated at a loss for much of its life, and it was the real estate development that kept the trains running, not the other way around. (Similarly, in Northern California, the Key System streetcars were actually owned by a company called the "Realty Syndicate".)
Finally, there's the technological question. One-car streetcars running in mixed traffic are technologically inferior to diesel buses, because they can't be rerouted for construction, and if something blocks the tracks, there's not much the streetcar driver can do except wait. In the Pacific Electric's case, many of their routes ran down busy thoroughfares, and they didn't have dedicated lanes the way that the modern L.A. Metro's trains do. And buses are cheaper to run per-passenger on low-capacity routes - there's no overhead wires or tracks to maintain. Trains really shine when they have dedicated right-of-way (i.e., their own dedicated lanes or tracks), and it's a high capacity route - a three-car train of streetcars, like the modern Expo Line, can handle 600 passengers per train with a single driver, which is equivalent to five buses. But one-car trains of streetcars running in mixed traffic have both the disadvantages of a train (can't detour) and the disadvantages of a bus (limited capacity).
Because of this it made a lot of economic sense to abandon streetcars for buses. GM and National City Lines took advantage of this process to sell buses, but it was a decades-long trend that began long before the alleged conspiracy began in the late 1930s.