In most of these cases, Christianity did not organically spread. It was brought in and forced upon local populations by mostly western colonial powers. Colonial powers always export their cultural beliefs and identities in an effort to bring those subjugated in line with their own ideas and expectations. Where religion wasn't brought in by colonialism's force, it was exported and introduced by typically western missionaries, yet another example of using a different kind of force to help it take root and spread. After some time, it's true that Christianity began to evolve with its own localized flavor, but it's not like these non-western cultures discovered Christianity via their own Damascus Road experiences with local prophets.