Earlier quoted context omitted.
Bus Rapid Transit is another option that could be amazing (while being much cheaper to implement), but it falls short for the same reason as trains: they require dedicated infrastructure that complicates driving, and complicating driving is political suicide.
BRT is mostly "you get what you pay for" - cheaper at a cost of lower capacity. Given relatively low density of US cities - that might be the right tool tho.
A BRT system would have had a capacity of, very optimistically, 6,000 people per hour per direction (a 100 person bus arriving every minute), but in practice probably less (that is difficult to maintain without delays). At the time, there was some doubt that even this capacity was required. The original tram setup also had a capacity of about 6,000 people per hour. Within years, the trams were full to overcrowding, and the line was expanded to 10,000 people per hour. Now it's at capacity again, with some of the longest trams in the world arriving every three minutes at peak. The only realistic option to further increase capacity is to turn it into a metro line (which, fortunately, was in principle planned for from the start, and _is_ possible, albeit with some disruption).
If they'd gone with a BRT, they'd now be looking at ripping up the whole thing and having nothing for years while they laid tracks.