Las Vegas was a water stop for steam powered trains going between Los Angeles and Salt Lake City.

When Boulder Dam was built, the government built Boulder City to house the workers. Being a government city, gambling wasn't allowed. But Vegas was just over the hill and filled that desire for the workers.

When the railroads switched to diesel and didn't need the water stop anymore, Vegas was already established as a gambling destination.

The spring still flows, and is part of a tiny state park in downtown Las Vegas.

And Boulder City is still the only place in Nevada where gambling is illegal.

From wikipedia, it was a waypoint between Salt Lake City and Los Angeles, then the railroad came and used it as a stop and the city built up around it. Then they legalized casinos and the mob took over.

a bunch of water and power right down the road in one direction, and a lot of population to attract to their gambling in the other?

Just a guess.

Aquifers.

IIRC Las Vegas was originally the company town for the workers building the Hoover Dam.