This is truly an excellent question to which there are numerous answers and numerous conspiracies. The first answer would be that in some jurisdictions, online gaming is purely illegal. In this case the credit card institution would obviously want to comply with the law. In jurisdictions where the law is grey, some credit card companies are afraid of lawsuits and negative PR. All it takes is one bad apple to sue the company and it can create havoc in the media, as well as drawing some very unwanted negative PR. For them its much easier to simply block transactions rather than deal with the fallout.