One factor nobody has mentioned yet, social stigma. Busses in the US are often associated with poor people who can't afford a car or have lost their licenses. Trams do not have the same stigma.
> Busses in the US are often associated with poor people who can't afford a car
Also, to be blunt, black people. I live in a pretty white area. If I didn't ride the bus, I could probably not see a single black person for a week or two at a time. Riding the bus, it's every day. And I'm quite sure a lot more people prefer the former scenario than will admit it (not that they necessarily have any particular animosity, but a lot of people are "just not comfortable" inhabiting the same space as black people).
In part because the races are taught different norms about appropriate behavior in public. The black people sitting quietly aren't the ones everyone else is hesitant to be near.
Also, to be blunt, black people. I live in a pretty white area. If I didn't ride the bus, I could probably not see a single black person for a week or two at a time. Riding the bus, it's every day. And I'm quite sure a lot more people prefer the former scenario than will admit it (not that they necessarily have any particular animosity, but a lot of people are "just not comfortable" inhabiting the same space as black people).