Because it depicts a group of people who, without hesitation, willingly caused incredible suffering across an entire continent. And they did so at a time that perceived itself to be the result of the enlightenment and humanism.
Emotionally, I can identify with people who feel that those who have proudly sown human misery do not deserve to rest in golden frames.
[edit] As OP mentioned, many have shown the other side of war; Grosz, Dix, Goya (https://www.theguardian.com/artanddesign/gallery/2014/may/14...)
I can emotionally identify with people who think that sowers of human misery are bad, but not with people who think art should be destroyed for what it depicts.
I can understand when someone refuses to address butchery with a polite and artful response.