Popular explanation but largely false. They were destroyed because people quit using them.
The story often goes that someone destroyed a fully operational system for profits when in fact the system was badly deteriorated and not really used a lot.
I'm not especially pro-suburbia--I live in an exurban/almost rural location. But I think it's perfectly understandable why many people wouldn't live in a city given a choice which they increasing had post-WWII in the US.
Seems like you'd be S.O.L. unless the government kindly free-markets a continent-wide network of paved roads, with conveniently placed roadside stops along the way where they sell gasoline. Can't imagine why they would undertake such an exorbitant public works project when the transcontinental railroad already exists; it's not like the government is disproportionately influenced by any industries that would very obviously materially benefit from such a thing.
I'm sure that Roger Rabbit might be more true for some cities, but I dislike that LA and other cities cling on to this. It excessively absolves everyone else -- politicians, press, homeowners, and PE/LARy themselves -- of their own fault. They could've saved their transit system like London did in the 1920-30s. Even today every SFR owner seems to prefer pointing at the Roger Rabbit story rather than looking in the mirror. Thank god for AB2097 and other new laws that address low density.
Sources -- basically any source that's not Roger Rabbit will tell you the same thing:
* http://scsra.org/library/rapid-transit-history/
* https://laist.com/news/entertainment/union-station-history
* https://www.vox.com/2015/5/7/8562007/streetcar-history-demis...
* This entire book; the preview pages have a lot of pertinent parts: https://books.google.com/books?id=OfTlph3cXoQC
Europe is not immune either, they would've followed the US's footsteps entirely if it weren't for the 1973 oil shock, which hit them harder than the US.