Google doesn’t want to dictate what is acceptable content on the internet. The people who buy ads don’t want to be associated with racism.
Honestly it would be better for this sort of thing to be banned from the top down by governments, but they seem loath to call any "white" nationalist group terrorists no matter how many weapons they bring to rally and how many people they injure.
White is in scare quotes, because you never know who they are counting aa such...