In order for the casino to attract new players, advertising is necessary. But if gambling is prohibited in the country, then advertising of such services will be prohibited accordingly. This is exactly what governments are pursuing in order to reduce gambling addiction among their citizens.
Banning gambling might help to reduce the gambling addiction, but how significant is always remain questionable. On other hand, there are always thousand ways to rome. It means gamblers who being addicted will always looking for a site to gambling. So they can gambling use abroad site, and adding more income to the other country.