One glaring thing i noticed that people have missed is that Yasuke had no last name (family name). In ancient times, Japanese peasants didn't have last names only the nobility and the samurai did. So the fact that Yasuke has no last name/family name and was never given one means he was never accepted as a samurai. (besides the fact that you know....the title of samurai was passed down through blood and you couldn't just become one anyways)
Of course this is just if Yasuke did exist, but he didn't.
It's about as common as a dirt farmer being elevated into nobility. Technically, it could happen. Did happen, with (I think?) Hideyoshi. But it's an incredibly rare thing, notable when it does happen. Add to it the fact Japan was in-fighting and in-warring for centuries, and any losing side lost all their nobility often, and what you get is a very slim pickings as to any common filth who ascend to nobility AND who live long enough to have three generations and a "real" family name to pass along.
Hideyoshi was elevated to the nobility yes, but out of a combination of being a very interesting person of his own right and Nobunaga's legendary capacity for doing things on a whim.
Both of them were chronicled in the Shinchoko-ki.