The Atlantic's CityLab blog looks at American streetcars and finds that infrequent service and slow speeds are their main problems:
Very few next-generation streetcar lines run with the sort of frequency that might counterbalance slow speeds or short distances. In a very smart post at his Transport Politic blog a couple weeks back, Yonah Freemark lamented that many U.S. streetcar (and, to be fair, light rail) systems built since 2000 fail to meet minimal service standards—often running just a few times an hour.
Good public transportation requires trains or buses to run every 10 or 12 minutes, five or six times an hour. Only two streetcars (Tacoma and Tucson) hit this mark. It's perhaps no coincidence that the brand new Tucson line also met its early ridership projections, even after ending a brief free-ride campaign and even before University of Arizona students were back on campus.
It's almost cargo-cult urban planning. Some streetcar systems—notably New Orleans'—work very well. So naturally urban planners saw streetcars as a way to improve central cities. But through lack of funding or lack of comprehension, they implemented the systems inadequately.
Of course, much of this comes from an inability to address the biggest issue with modern American infrastructure: we beatify cars. Maybe thinking about how to get people from one place to another without cars might help.