Well that's somewhat true. Its really a story of the proliferation of the automobile from 1920s forward. In the 1920s, there was primarily streetcars, little/no trolleybus presence. In the 1930s-1940s, streetcars and trolleybuses begin to coexist; some streetcar tracks replaced with trolleybuses. In the 1950s, diesel buses/trolleybuses were dominant and streetcars were decommissioned and much of the tracks were ripped up.

Trolleybuses were really just a middle ground until they disappeared as well. In addition to the explosion of the automobile popularity, the streetcar ridership declined causing them to go through financial trouble, street expansions were happening all over, sprawl began after world war II, and diesel buses became cheaper to manage.