Tyler Cowen ponders why people like streetcars so much. Possibilities offered by him and his commenters:
2. Dedicated lane
3. Less noise
4. Less swerving/lurching (presumably, also less carsickness)
5. More predictible (no one suddenly re-routes your streetcar)
The streetcars I'm most familiar with are Philadelphia's, which fail many of those tests, so let me offer my take on why people (read: affluent, especially white people) like streetcars: they don't have so many poor people on them.
Streetcars are developed in a fixed area and not frequently expanded. They have a high capital cost. This means the area along the lines gentrifies. Thus, when you get on a streetcar, it normally has a lot of other affluent people on it. By contrast, when you get on a bus, it normally has a lot of poor people who have been sitting on it for an hour, patiently waiting to get to work. The association builds between streetcars and affluence, busses and poverty, in one's mind.
Problems with this theory: trains in New York are usually crowded with poor people from outlying areas; wealthier people use them anyway. On the other hand, it's basically impossible to either park or drive a car to work in New York; it is frequently possible to walk across town faster than you can drive during the day.