Because they continue to post in a way that would alienate all but the most combative of contributors.
If you are going to post a topic which has the word 'F*ck' in the title it can't come as a surprise that many people won't be interested in taking part in such a thread. If you post that you don't think you should use the word c*nt as a direct term of abuse, but you wish to reserve the right to use it in general on here, then again, why be surprised that that's a style of posting that will deter most newcomers?
Long before the recent outbreak of sweariness it was clear that NACA doesn't attract many new members. People might say they would welcome more diverse viewpoints but I'm not sure they actually mean it.