Fundamentally because they're on the same contiental plate, and had exposure to the same germs, having been infected by the same or similar animals. The decima

Fundamentally because they're on the same contiental plate, and had exposure to the same germs, having been infected by the same or similar animals. The decimation of the Americas was largely due to the fact that the ships that arrived brought with them germs to which the natives had no defences at all. This topic is covered extensively in Jared Diamond's Guns, Germs and Steel.