Yes. Luck. There are a few other posters in this thread that objected to Diamond because he seemed too Euro Centric apologist, the argument being "well why didn't the East win". But sometimes it does turn on a flip of a coin. The Mongols (eastern power) could have won, and dominated, and colonized the world. And we would all be studying how horrible Mongol Colonialism was. Sometimes history does swing on singular events.
>And we would all be studying how horrible Mongol Colonialism was.
that presumes that human culture would have developed the ideas regarding colonialism which we have and which after all are a late developed critique that would probably not have been possible without the ideas of humanism and similar intellectual branches, and the exposure of those ideas to people who were being oppressed by forces that claimed to espouse the same.