It's pretty simple. If you give racists a home on your site, they will start congregating on your site (because nobody else will let them post their openly racist bullshit). The thing is, they don't just stick to the explicitly racist forums, they'll start using the whole site, as any other user. When you have a sizable number of racist users, submitting links, making comments and voting to increase the visibility of content as an organized bloc (remember, all links & comments on Reddit are pushed to the top based on user votes, not admin/moderator/editor fiat), the result is that the entire site, even the "non-racist" parts gets regular doses of white supremacist propaganda.
The core demographic of Reddit, young middle-class white males, is exactly who white supremacists want to recruit. They're naive enough to buy into the "oppression of the white race" narrative that white supremacists continually spew.
To put this in perspective, it's been estimated that Reddit's racist subs currently have more traffic than Stormfront - historically the center of online white supremacist activity.