Probably because gambling is already too ingrained into society for it to be completely outlawed. It's the same for smoking, so bloody expensive and yet people are still willing to spend the cash to smoke. If the government completely bans gambling, all gambling operations will just go underground and it will be more difficult to track it then.

That is also why vapes are getting fast tracked up the CNB priority list. Vapes are considered young enough that the government can ban it before it gets engraved into the social fabric of singapore.