I sort of see what you mean, but characters like Nightmare, Siegfried, Mitsurugi, Taki, Sophitia and the like are hugely iconic to the series in their own right. I think it's more like they don't want to go through all the extra effort in adding it. After all, Tekken is the game that gets treated like some kind of masterpiece that doesn't need any major changes, hence why we don't have a cool feature like that. Soulcalibur has more room for that kind of stuff, because it's the game that gets less attention from Namco so they're free to do whatever. It seems that Tekken remains comparatively stagnant, but Soulcalibur can be more experimental (not always a good thing though, look what happened with SCV).