Well they get their water from the same source (Colorado River). The difference is that river is right next to Vegas, so it's not imported. San Diego pipes it 200 miles across the state.
> The reason Vegas has such limited water is because California and Arizona have far more water rights.
Which is a key reason why California needs to be sticking oodles of desalination plants along its absurdly long coastline yesterday. There is zero good reason why a top-10-in-the-world economy concentrated primarily on said absurdly long coastline should be dependent on water flowing from literal deserts.
If California's so averse to the requisite power requirements to make droughts a thing of the past (it's almost as if shutting down nuclear plants might have downstream effects), then there's ample opportunity for Nevada and Arizona to step up to the plate as literal powerhouses. On geothermal energy maps, Nevada in particular is a giant blob of heat; plus, I can think of no better place for sprawling solar farms than "inhospitable" deserts. Nevada/Arizona powers California, California hydrates Nevada/Arizona, win-win.
The reason Vegas has such limited water is because California and Arizona have far more water rights. (Only 4% in fact) https://en.wikipedia.org/wiki/Colorado_River_Compact