The zionists (and secular Jews in general) believe that the goyim are inherently good and worthy of emualation and being assimilated into, and therefore have to hold that the holocaust was something radically out of the ordinary. Therefore it rates a special day, and much special attention.
A frum historian will more likely come to the conclusion that the holocaust wasn’t all that “special”, and was merely a mid-20th century manifestation of what we have seen for many, many centuries (and millenia), going back to the origin of Tisha B’Av. If we had a full day fast every time the goyim persecuted us, we would risk starvation.