I don't know but last I glanced at it, the Unification was implied to be coming from Chinese pressure than Japanese. Each of Kanji/Hanzi countries already had each own acceptable common use character sets and those could have been just populated into each own spaces. That seemed like the original plan at some point.

There has to be a reason why it happened(as to why Unicode suddenly started insisting it has to fit inside a 2^16 total chars or whatever).