I also think that after working for years on game that was – in all fairness – concentrated mostly on Europe, and having done coronations in some forms in CK1, the team is much more familiar with the Coronations subject matter than it is with AuH.
Of course, PDX has resources to consult people with greater familiarity with the matter than the overwhelming majority of forums has… but still: at this point they probably are very familiar with the feedback over how the Europe should look like & what the players want (even if they will make an informed decision to do otherwise), yet never really received any feedback to what players would like to have implemented in terms of CK3 mechanics for Japan or SEA.