I can't say this for sure, but I think most of it is down to regulations. Regulations encourage the eating of beef, chicken and pork but discourage the eating of almost ever other kind of meat. Let me explain. Many people hunt deer, but almost no one is allowed to sell their deer meat. You cannot go to a butcher or a grocer and request deer meat. For that reason in spite of its plenty most people in the US have never tasted deer, but most people have tasted beef. Horse meat regulations, similarly, make horse meat difficult to sell and obtain. It is put into dog food, but they won't sell it for human consumption. Therefore few have ever tasted horse. You'd have to buy a horse and slaughter it yourself to have horse meat.