Gotta be someone who lived/worked in the area and wanted a hidey-hole. You could wait out the initial chaos for however long it took (1 year? 5? 10?) before you came out and headed to your second, longer term spot up the Rocky mountains or whatever. When you have the money for a $5m bunker it just all becomes logistics of prepping and storage.

After TEOTWAWKI Vegas will probably empty out pretty quick without power or irrigation, it might be a nice safe spot to ride it out if you’re prepared for it.