Most of the US's best MMA athletes tend to come from wrestling and a low center of gravity is a huge advantage in wrestling. To reach the highest levels you typically need every advantage you can get so it's no wonder the guys who have the right body type tend to see more success. I don't think it's really an American thing so much as it is a sport specific trait that allows for efficient movement. If you ever watch international Judo, Greco Roman, or Freestyle wrestling you'll see that most guys are of the same build, it's a very common theme. If Muay Thai were as popular in the states as folkstyle wrestling is, you'd see a huge shift in body type that transition from that sport to MMA.
You can look at swimmers and see that they all have very long torso's too which allow for more surface area/buoyancy when swimming. Top sprinters/runners tend to have very long legs in comparison the rest of their body. Gymnasts are usually short as fuck with short arms which allows for more leverage.