In part because America is much larger: NYC to Los Angeles is similar in distance to a trip from London to Baghdad. At that scale, trains are always going to be inconvenient.
Now, that's not to say trains in the US are good, but they are never going to be the best form of transportation for a 3,000 mile trip.
Now, that's not to say trains in the US are good, but they are never going to be the best form of transportation for a 3,000 mile trip.