The citizens of Monaco are not allowed to gamble. However, only something like 21% (or 8,000 of the 38,000 residents) are citizens. As for why those 8,000 citizens are not permitted to gamble, a few sources stated it was a moral policy that dates back to the original establishment of casinos in the 1850s.

Monaco Gambling Laws in 2024\n\n But there are some catches. If you visit Monaco, you will be allowed to enter these casinos  sometimes paying a fee  but local people are only allowed to gamble online, meaning you won't be mixing with any the country's residents at the blackjack or roulette tables.