I know LA has its Metro service and there are a few light rail systems, but why not a heavy rail system like BART? When the commission that ultimately led to BART was created in the 1950's, the Bay Area's population was just over 1 million. The commission concluded that traffic would get so bad in the coming decades that traffic would be at a stand still. The population then was growing by about 1 million per decade. LA has terrible traffic by comparison, so what gives, it would seem that LA should have gotten its own system after BART was built.
There are 2 heavy rail system in LA, the Red and Purple line compared to 5 BART lines. There are 8 Metrolink lines in LA compared to just one CalTrain line by the Bay. And SF has 7 Muni light rail lines compared to 4 in LA.
So it isn't true that SF just has one BART system. You might be surprised how many rail miles and train stations LA has compared to SF.
LA miles, stations, and ridership by train type:
Light Rail = 70 miles, 69 stations, 357,000 total
Heavy Rail = 23 miles, 22 stations, (")
Commuter Rail = 388 miles, 55 stations, 42,000
Total = 481 miles, 146 stations, 399,000
SF miles, stations, and ridership by train type:
Light Rail = 34 miles, 33 stations, 87 additional stops, 150,000
Heavy Rail = 104 miles, 44 stations, 374,000
Commuter Rail = 77 miles, 32 stations, 47,000
Total = 215 miles, 109 stations, 571,000
Believe it or not, Southern California has more rail miles and train stations than Northern California. Maybe that's the result of not building as much more expensive Heavy Rail. But Northern California has more weekday riders today - it'll be interesting to see how LA ridership grows as they expand all their rail systems.
As for why they didn't copy BART as much; they don't have SF Bay to run tracks under.