Human migrations were at a snail's pace. In Europe, for example, for literally centuries, villagers were born, lived and died without ever traveling farther than ten or twenty miles from home. Oceans, mountain ranges and mere miles kept us apart, and that very isolation reinforced our local religious beliefs. We were never exposed to anything different, everyone we knew believed what we believed, and this went on for centuries. When cultures finally did collide the result was usually war.
History is a depressing, dreary litany of wars.
Humans are basically conservative and don't like sudden, extreme change. We fight back when we meet something alien. But now think about how quickly all that is changing. In one generation Japan has transformed, almost miraculously, from our hated, fierce enemies, to a country we can barely keep up with technologically. In just one decade home computers spread like wildfire.