Reasons stated above as well as the fact that this is something people want but nobody wants to pay for. We could have a really decent “medium” speed rail (79-90 mph) system quickly and cheaper than some of these high speed rail proposals, but people aren’t interested in that most places and public officials know that. Amtrak is basically an extension of the federal government and most of their system is simply just there to say that they provide a service or provided because of state subsidies (Oklahoma pays for the Heartland Flyer). There’s a lot of politics involved at all levels and it’s one giant mess to which there is no reasonable solution barring any sort of major upheaval in the political climate, economy, or public perception.
On a similar note - look at the mess that is US 69 between the Red River and McAlester. The amount of traffic that road carries is insane, yet it basically runs through the middle of every town (except Durant) like it always has, with all of the attendant congestion, slow downs, stop lights, etc. It really should be upgraded to Interstate standards, but the Feds won’t pay for it, ODOT is busy trying to push through a bypass of Muskogee, and every city, town, & wide spot that collects speed ticket revenue fights any proposal to do anything about it.