I live in Boston where our subway transit system still has a branch of streetcars. The points about passenger density and right-of-way are spot on. Actually the only thing that holds up the streetcars are when drivers are acting stubborn and entitle themselves to right-of-way...or when mother nature dumps 3 feet of snow on the city and the old transit system can't keep up.

But everyone still uses the streetcar system because you can get from point A to B, no matter if B is half a mile or 5 miles away, for one fee of $2.10 vs ride-sharing or taxis which accumulate charges over time and distance.