The Colorado has almost 250,000 square miles of basin. The Chattahoochee less than 9000. Atlanta metro is about six million, Las Vegas metro about 2.5.
Politically, the Colorado is a different animal. Mexico is downstream and historically has just had to lump it when the river runs dry. Which it has as the US built dams. And today, it has to make do with just a remaining 5% of the historical wetlands where the Colorado flows into the Sea of Cortez.
Las Vegas got to take advantage of an early 20th century hydroelectric dam project. They also had frequent water restrictions when I lived there in the 90s, and the population has only grown so I imagine it isn't much improved now.
Atlanta's artificial lakes/reservoirs are from a similar time, new ones aren't being made fast enough if at all, and the population continues to grow. Dealing with multiple jurisdictions (the region known as Atlanta encompasses the city of Atlanta, many other cities, and is spread across several counties) further complicates the issue when it comes to funding and determining responsibility. You could argue the state of GA should step in. However, there's a huge political chasm between the metro Atlanta area and most of the rest of the state (A "fuck those city slickers" attitude is very common in rural Georgia and the smaller cities, Atlantans aren't viewed as Georgians given how many are transplants). The water in the Atlanta region also needs to travel down river to many other areas (and other states) in order to sustain those regions.
It's just a One Major Metropolis versus rural thing. Chicago is Chicago, not Illinois. Watch two Illinoisans try to sort out they're "From Illinois" but one's from anywhere but a Chicago exurb, and they'll say, "Oh, you're from Chicago," not, "cool." More like, "Yeah you're technically in Illinois ... You're in the corner by the weird lake and you're way too close to Wisconsin and Indiana for comfort anyway."
You don't see these problems in states where the the one major population center that happens to control the state government isn't seeking to exert strong control over the rural areas.
I was last in Las Vegas 10+ years ago. The shrinking of Lake Mead, the reservoir made by the Hoover Dam, was alarming back then. I'm afraid to even check it's current status.
Cheap solar in the west (California, Arizona, Nevada) coupled with desalination plants. We'll be fine for *urban/residential* water needs for the foreseeable future.
Politically, the Colorado is a different animal. Mexico is downstream and historically has just had to lump it when the river runs dry. Which it has as the US built dams. And today, it has to make do with just a remaining 5% of the historical wetlands where the Colorado flows into the Sea of Cortez.