You can have a normal thread where someone will casually drop the N word or other slur and it will just sit there, maybe not even acknowledged but also not downvoted or deleted.
That alone is enough to turn people off immediately. They need that sense of retributive justice for a wrong they see in the universe. You're right that reddit can be just as bigoted than 4chan if not more so but I think you realize it's way more subtle and dogwhistley. The only reason a 4chan post will ever dance around being blunt bigotry is usually for the sake of humor not to ban evade.
Also I can't imagine browsing 4chan without something like 4chanX and your average person probably isn't going to bother installing that.