That's because Namco wasn't focusing on the culture of the characters (they never were). Like i told OP, they can't just start throwing in characters from a bunch of other countries, because they're going to end up with a lot of characters with little to no relevency. Representing cultures isnt what series does and they wouldnt do it well if they tried. Believe it or not, no one is going to buy Soul Calibur because a character came from their country. If they did, we wouldve seen the French players main nothing but Raphael and Amy and the Japanese just maining Mitsu, Taki, and Yoshi (and maybe Sets too).
The last thing Namco want to do is stir away from SC being about weapons to become about warriors from 100s of different cultures just to appeal to a different crowd (tactics like that can kill a franchise). It wouldn't be SC anymore because the Soul Swords wouldnt be the main focus. If anyone wants all those different warriors more than anything else, then give Deadliest Warrior a try.