J. Robert Oppenheimer, the high-powered physicist who became the scientific director of the Manhattan Project, loved New Mexico and had even bought a ranch there.
When he was looking for a remote place where secrecy could be maintained more easily, a New Mexico location seemed ideal.
Indeed, Los Alamos is located on the former site of a private school that Oppenheimer was familiar with.
The scenic surroundings would also be a plus in attracting top personnel, although for most people that would have been a distant consideration to the sense of existential national threat.
At the beginning of WW II national priorities were somewhat different than today’s!
Even if the dangers of radioactive materials had been fully appreciated, they would’ve taken a back seat to the sense of national emergency.
Such attitudes lasted well into the Cold War, and indeed have not vanished even today among the personnel who work at Los Alamos and allied places.