Price and permission from the states that own the rights to that water are the two big reasons I can think of off the top of my head, with price being the biggest obstacle. Most of the areas that need help are not only a long way away, but there is a significant elevation difference. For example, Amarillo is about 3,700 ft above sea level. The Mississippi River in Memphis, TN is at about 200-250 ft above sea level. You would have to move the water about 650 miles horizontally and about 3,500 ft vertically. The cost would be enormous just to get the water from point a to point b and that doesn’t even factor in the cost to build a massive pipeline. And that is only one area that needs help.
The area mentioned in the op is a little farther away (~700 miles) and a little lower in elevation (~2,500 ft above sea level).
Peace
Tim
I’m sorry. I didn’t mention Amarillo. Nor did I mention Memphis. I mentioned the Ozarks and Dallas. Dallas is about 425 miles away by road from Branson, Missouri, where there’s a lot of water. As the crow flies, it would be less. Branson is about 300’ higher in elevation than Dallas. The Springfield Plain, where much of the water comes from is higher still; about 800 feet higher than Dallas/Fort Worth.
Amarillo will need to find its own water. I can’t help them.

Might not be anything anybody can do to improve the water situation in the Llano Estocada. The rainfall at Dallas isn’t all that bad, but having said that, I’ll bet there is a water table “dimple” under and around Dallas that would cause you to catch your breath, it would be so deep.
But I will say this. If there was some way to carry off flood waters in the White River basin or in the headwaters of Grand Lake in Oklahoma, I’ll bet there would be no argument from the state authorities about it.
Well, then, KC is farther away from Dallas than the Springfield Plain, but since St. Louis is not much higher than Dallas, forget the Mississippi. Missouri River water could be sent to Dallas from around KC, which is about 600 feet higher than Dallas. The route wouldn’t be so far away from Springfield or Branson that a “feeder” line couldn’t be extended to it.
But you really don’t need enormous differences in elevation for water to flow. I likes to go downhill. Most of the Roman aqueducts sloped very gradually, almost imperceptibly.
Truth be known, I’ll bet it would be no more difficult to do that than it is to send most of the Colorado River to L.A., Las Vegas and all the irrigated farms in the California deserts.