Ok, so you get in a train that drops you off in the middle of LA. Now, how do you get to where you're going from there? LA is hundreds of square miles of urban area, all spread out so there's no way any train will take you to all parts of it. You'll need a car to drive yourself to your destination. Now you're looking at spending a bunch of time and money dealing with a rental car agency, instead of just using your own car to get you there.
Trains are just like planes, only a lot slower. Planes are great for getting a medium number of people between two points all at once, in a short amount of time (except for TSA groping). But they don't help you much in getting from the airport to your final destination. Trains are worse because they're so slow, it ends up not being sensible to use them too much because if the distance is short, you might as well drive, and if it's longer, you're better off flying. If you happen to live in an urban downtown and want to travel to another urban downtown not too far away, trains make a lot of sense. That's about it though.
What would make a lot more sense is if they'd build SkyTran, but no one believes that'll possibly work so we can't have it.