At the moment there is an ongoing academic debate about the Holocaust, or actually about how it is being understood, taught, commemorated and remembered. In other words, it is not about the past in itself but about history - not about what happened, but about our retrospective engagement with it.
The basic question in the current debate is whether the Holocaust was "singular", that is to say: historically unique, or actually a part of a much longer historical trajectory or "direction" other than Antisemitism. Some historians argue very passionately for us to think of the Holocaust as a result of other directions that aren't specifically Jewish, focusing particularly on Colonialism: The argument is that Europeans had been carrying out many genocides, so the Nazis could be understood as a radicalized continuation of that European (and also German) legacy, i.e. as having imported this already existing practice back to Europe. Indeed, there are for example biographical links between the German colonial practices in South-West Africa (nowadays Namibia) to the Nazis and their practices.
This is, however, not the first debate about the Holocaust's potential or actual singularity - in the 1980s, a very similar debate took place, except back then the broader context was not Colonialism but Stalinism: Since the USSR had been incarcerating, enslaving, starving and murdering millions in camps and the Nazis understood themselves as a reaction to the "Judeo-Bolshevist" danger, the Nazi practices could be understood as a radicalized continuation of the already existing Soviet practices, in other words: as Hitler's attempt to "out-stalin" Stalin in the latter's own game.
In fact, it seems that in any way one looks (back) at the Holocaust, every aspect of it can be traced back to some historical precedence. Be it mass shootings of tens of thousands, the elaborate galaxy of camps to "process" millions, or the use of poisonous gas to kill human beings - each of these had actually happened before, just not in the particular combination the Nazis created. So when we consider the many trajectories that the Holocaust can be placed on, maybe the Holocaust was just more-of-the-same: perhaps quantitatively, but not qualitatively different; worse, yet not unique.
So far our observations of the academic discourse at the moment. And now, we would like to suggest that there is actually no contradiction between a "multi-directional" understanding or broad contextualization of the Holocaust on the one hand, and its singularity on the other.
Indeed, the reference to historical developments before the Holocaust - be it Colonialism, Stalinism or in fact both as well as others - is important for understanding various aspects of the Holocaust. Without including these other directions, it's impossible to provide a functional explanation of the Holocaust. Thus, placing the Holocaust in longer historical trajectories really does help us to understand how it could be done - and yet, it doesn't address the why.
In terms of why the Holocaust was done, Antisemitism - or bluntly put: the idea that the world would be a better place without the "oppressive" or "exploitive" People of Israel - remains the core issue. This very specific context is ultimately the essence that (at least in our opinion) makes the Holocaust historically unique and, indeed, singular.