I'll try to answer tough I don't have exactly official answers, I did read the answers through the threads.
They don't allow playing the Irish or Wesh because:
1. They didn't hade enough historical significance in the medieval world (this is not an opinion of mine, its just what is said);
2. There is no room for them, because more important faction already exist and because there is a limit of faction to the game (why I don't really know);
I don't really know why you can't play the Kingdom of Jerusalem.