I reckon the tunes were mostly written by people who wanted to make it easy for others - perhaps not particularly good musicians - to play them, so it was more accessible to playing in every little village/church hall/pub, so that folks could dance to them. After all, they were mostly written as dances, and, from watching 'Go west along the road' on the now-defunct Tara channel, local people were always playing and dancing to them. So, simplicity for accessibility is my opinion.
Thank God! It's hard enough playing in those keys, without E flat, z sharp minor etc.