Final answer:
The transition from multiple competing city bus companies to usually one subsidized, regulated monopoly in major U.S. cities was likely due to efficiency, scale benefits, and governmental control preferences.
Step-by-step explanation:
In the mid-twentieth century, multiple city bus companies were common in major U.S. cities, yet today this has changed to typically only one which operates as a subsidized, regulated monopoly. Several factors may have contributed to this shift, such as the need for more efficient coordination of public transit services, the benefits of scale in operations, and the governmental preference for control over the quality and pricing of public transportation services. Financial viability also plays a significant role, as competition could have led to unsustainable pricing models that would not cover the costs of running such services. The mentioned 381-day boycott in Montgomery and other historical activism further impacted public transportation policies and practices.