Maybe someone stashed those there before the war in preparation.
Gambling was legalized in Vegas the same year Hoover dam was started (1931)
If Hoover dam was built in the 30's and the atom bomb was not developed until the mid 40's then the dam would have been there before the war (and the cave would have been underwater). Hoover certainly could not have been built after.
Even assuming a different timeline, a structure that big could not have been built after a globally devastating war.
So this skeleton would have to be someone who was killed and dumped there along with all his stuff. My guess is the White Glove Society wanted him/her to "disappear" with all their belongings (minus the meat) so they dumped the remains there.