I believe that ISO2022 allows for the full set of japanese names (and has some sort of process for introducing new kanji). That's probably a big part of the reason that Ruby's strings are bytes with an encoding attribute, rather than just being unicode.
Ruby was designed as utility goop for Japanese programmers. Inability to parse/output legacy encodings would have rendered it virtually useless for that, even if legacy encodings were strictly dominated by any available Unicode encoding, which many Japanese programmers would hotly contest.
The Ruby thing is probably due to the fact that EUC and Shift-JIS were then (and to some extent still are) the prevalent encodings. It's not so much about character sets, after all Unicode includes every kanji defined in ISO-2022. Please see my other comment in this thread.
