Huge advantage of amtrak is that the stations are in prime locations. In the northeast that's a huge advantage as going from train to destination might be a 15 minute walk. Meanwhile airport to destination could be an hour drive. Sunbelt cities have no density, so the trains location isn't as valuable.

Development and land use patterns are way different in the Sunbelt. There are lines that go through most downtowns but most of the people live far from the stations. The passenger trains also share lines with freight and freight is prioritized, a trip from Austin to Dallas by Amtrak takes 6 hours but can be driven in 3 or so.

Lots of people in the Northeast don't live in downtowns either. One big difference if that, if you're taking a trip to one of the big cities, you often don't need or even want a car when you get there. On the other hand, if you can drive somewhere in 4-5 hours, taking a train and then having to rent a car when you arrive seems a lot less attractive.