Before you say 'money', my experience is that DEFCON attendees are very unlikely to gamble (the main reason to host conventions). Plus all the collateral damage: I remember the Starbucks POS systems at the Rio would always be hacked during DEFCON and other guests couldn't work out why the wifi didn't work
You're just hanging out in the wrong crowd.
Hence they tend to frequently change hotels. Some of the hotels (Alexis Park) would charge exorbitant repair fees for the rooms, and probably make at least some money for their trouble on the side.
Although some people just go too far, like throwing lit road flares into the elevators and sending them down to the casino floor.
They really do try to keep things in check though, as best they can.
Oh, and alcohol sales.
Probably because Defcon pays them a pretty penny to host it there. I'm pretty sure the old tactic of subsidizing events to get people there hoping they will gamble has been done away with. The $5 buffets and $20 Cirqu Du Solei' shows of the 80/90's have been replaced with $20-$40 buffets and $100 Cirque Du Soleil shows.