[ghaff]

Because all the US passenger rail companies went out of business. Amtrak was essentially created out of the wreckage to maintain some passenger rail service. (It's a very complicated history.)

Amtrak makes almost all of its money on the Northeast Corridor which it then basically throws away in the rest of the country.

[jeffbee]

Arguably Amtrak was intended to dismantle the remains of passenger rail, and its persistence 50 years later was not foreseen.

[ghaff]

I don't know. Passenger service collapsed in the decade leading up to the formation of Amtrak. Other than bailing out and heavily subsidizing a bunch of failing railroads, I'm not sure what the fix was--although the Federal government could have made a much bigger investment at the time than it did. (Basically, spend what it would take to create popular Shinkansen routes.)