Answer:
Africa and the Americas didn't have the same geographical and ecological advantages as Europe had.
Explanation:
According to Jared Diamond's book "Guns, Germs and Steel", Europeans invaded Africa and the Americas, and not the other way, because Eurasia (the combined European and Asian landmass) offered significant geographical and ecological advantages. Africa and the Americas run along a north-south axis, which means there is significant differences between climates along them. This prevents crops from an area of these continents to be easily grown when moving further north or south. Africa and the Americas are also notable by having major geographical features that make travel along the lenght of these landmasses quite difficult, most notably the Sahara Desert and the Central American jungle respectively. Finally, both continents lack easily domesticated species that could be used as pack animals, which meant native Africans and native Americans had to move on feet almost exclusively.
Conversely, Eurasia runs along a west-east axis, which meant crops grown in the mild Mediterranean climates could be grown all along this axis and support larger populations. Also, the heart of Eurasia is crossed by large swathes of open terrain, which made trade and communication easier all along the length of the continent. And last, domesticated pack animals, such as horses, donkeys and camels, enabled very fast travel in Eurasia.
All these factors combined meant that Eurasians, and Europeans in particular, had a technological edge which Africans and Americans could not match, enabling Europeans to expand across the Atlantic.
