In fact it's illgogical for the site to lose money as they have a house edge.
What can happen though is seeing the site running with your invested money :/
It happens though. The site could lose money if there have been more gamblers that won than what the casinos have earned. I've seen it in bitsler when a gambler was able to win multiple 4 btc wins and pretty large wins like 20btc. Which dropped the global bankroll by more than 30%. Though, it recovered the losses and it's in profits now.
Its really strange to know that gambling sites went in lose. Gambling is one business where only the player lose and the gambling site owners always win. Spending too much on advertising that make you not earn profit is not a good sign of business oriented mind.