American football is, as others have said a very complex sport. More importantly, it is very expensive to properly equip and train an American football team. The minimum effective size for a squad is 44 people, 22 each on defense and offense, allowing for one replacement player per position. The uniform involves very expensive pads and helmets, all fitted to each individual player. And, finally, because of the extreme specialization US football demands of its players, the range in size, strength, and stamina varies greatly. In places where the sport is unknown, it would be virtually impossible to start building interest in youth clubs and schools.
Baseball, on the otherhand, requires no special physical attributes of its players than good hand-eye coordination and a reasonably good ability to throw, catch, hit a ball with a stick, and run in short bursts. Normal people can play baseball, and the basic rules are much simpler to learn and understand. Like chess, the "deep" rules are very complex and sophisticated, but the game can be learned at the basic player's level very quickly.
However, I would take issue with the initial post's basic premise. Other than Japan and some Latin American countries, professional baseball is a very minor sport, although there are youth leagues in many other places. US football is popular in Mexico, and Canadian-rules Football is almost entirely the same game. US Football also seems to gaining in popularity in Europe, but this may be more for its spectacle rather than sport...rather like Ultimate Fighting Championships.