There could be a few different reasons. Such as the laws of those countries that are being excluded. Some countries may require competitions to meet certain standards or be registered in those countries for prizes to be won by its citizens. Registering can sometimes be expensive and out-weigh the prize value. Some prizes that exceed a certain value may not be able to be delivered due to import duties or taxes or it might fall on the winner to pay them. Sponsors too can have a big say in which countries the prize should be offered to.