More to the point, you've assuming the conclusion. If we could distill and pump water from the Pacific Ocean to Arizona at cost-effective rates then yes, it would be a significant technological innovation.
But knowing that it would be a significant technological innovation doesn't make it happen. We don't have many flying cars, even though the technology has been around for decades. Yet cheap flying cars would be a significant technological innovation, yes?
We already have the ability to transport Pacific water from the Carlsbad Desalination Plant in San Diego to Arizona, via the road system. If your argument has merit, then we should be seeing the existing price getting lower, right?
What isn't the price going down fast enough to meet the obvious demand?