The reasons for that is simple: the gamble regulatory policy may not be in line with what the casino providers might be watching out for as part of their objectives for their business.
Another thing is that, some casinos might out of owners personal judgement want to stay off from operating in certain continent so it may not be just your country alone for some casino's the restriction could go continental.
Lastly, some casino's might have a sister gambling site or casino already existing in your country and as a result they don't want to double there to avoid double taxation and also not to be a competitor with it's sister site in that same country.