On the one hand, one can understand the governments of those countries that ban gambling, although, as far as I can see, betting is not banned that often. Gambling can be quite unruly, especially in poor countries, and this leads to increased crime and domestic violence. Although it should be noted that betting itself is not much different in properties from gambling, but for some reason, in many countries it is not prohibited.
On the other hand, gambling is a great stress reliever. It is also a place where tourists can spend a lot of money and the government can collect pretty good taxes.
A government is certainly aware of the sector potential to generate significant revenue from gambling taxes. However some countries that have banned gambling may consider norms or religious majorities as the reasons for their prohibitions. Unfortunately the ban is not accompanied by a monitoring system, so those who still want to gamble can continue to do so, albeit illegally. This, of course, comes with the risk of violating regulations in certain countries that ban gambling.