There just aren't that many available water sources to draw large amounts of water from, and El Paso is going to be pretty much at the bottom of the list when it comes to irrigation supply. The Colorado is already oversubscribed, and tapping new water sources creates political backlash from locals who don't want to see their water going to other counties or states. Finally, pipelines are expensive beasts in a time when the federal government isn't interested in eating the costs and state governments can't easily wait for counties to pay back out of water sales.

Water is a bit odd in that it's ridiculously cheap even where it's scarce, so the financial incentive to build massive water-diversion projects isn't all that great absent a specific political or governmental imperative. It's also dubious that it's worth diverting water to places that are naturally scarce (or where wasteful policies have drawn down aquifers and other groundwater sources). Pushing the carrying capacity of a region well beyond its natural levels is a fraught enterprise, especially if the regions haven't shown themselves to be particularly good at managing their current water stocks.

Personally, I'm betting that in 20 years the Great Lakes rust belt is going to be enjoying a hell of a resurgence.

This comes up every time there is a drought in the south of the UK compared to the north. It really is hard to mentally comprehend the colossal amounts of water that a river is moving.

Water has a mass of 1 g/cm^3, or 1 kg/L, or 1 Mg/m^3.

Covering a 440-acre farm (about the median farm size in the US) to a depth of one inch requires over 44000 metric tons of water.

Oil pipelines have a transport capacity of millions of metric tons per year. A water pipeline of the same order of magnitude in capacity could give 227 farms the irrigation water equivalent of just one extra inch of rain per year.

The energy cost of moving that water downhill is actually negative. But rivers already do that. So your pipeline is probably moving that water uphill. Using the approximation that lifting 1 kg to a height of 1 meter is about 10 joules, lifting 10 million metric tons 100 meters is 10 Tj, or about the output of a tractor-trailer engine, running continuously, all year long.

Pumping it up from the elevation of St. Louis to the elevation of Denver is about 1500m vertically, and costs about the output of a diesel locomotive, running continuously, all year long.

That commodity typically costs $0.40 per ton in the US, only after it is completely processed for potability and delivered to the customer. Agricultural users expect to pay less.

This is probably well within the realm of our current technological possibilities, and may even be profitable economically, but you cannot even approach this politically. The cheapest way to move water is therefore to foment an armed rebellion with water rights as the casus belli. Then you can install an all new regime which likewise cannot be changed because too many millions of people depend on it remaining the same.