Why is Evelynn's E physical damage?

Evelynn is officially listed as an assassin, yet her E being physical damage directly conflicts with that role by making magic penetration much less effective on her. If it was magic damage, AP Evelynn would be much more viable while not really hurting AD off tank evelynn either since no one ever builds armour pen except for warrior on Evelynn since the majority of her damage is still magic.

I noticed in patch notes from a while ago that it in fact used to be magic damage, so why change it? It just makes her build order wonky because penetration became inefficient, even though a champion has to be able to make good use of penetration to build damage without being easily countered by resists.