There are several reasons:
It may be...
- the casino or the game provider/supplier is not up to date with the rules in your country;
- that they have suffered some sanction from your government and are banned/prohibited from operating in your country for breaking some law;
- that the taxes paid in your country are very high and it is not profitable for them to offer these bets in your country;
- that they have some prejudice and simply don't allow it because they don't really want to.
Anyway, I believe that the only way for you to get a conclusive answer, if you really want to know why, is to ask them through the customer service/support channel.