By that I mean not that expressing the meaning of a word is forbidden, not the idea, but the utterance itself. If, like me, you tend to assume that cultures become more progressive and tolerant over time, this is an interesting very strong counter-example. Having a word that you that literally can't say even in quotation or reference implies something almost akin to magical thinking. As if the two syllables themselves, even when stripped of meaning, still have some kind of power.
What I think is happening is that (like almost everything) it comes down to group membership signaling. Progressive, anti-racist US culture has decided that not saying the N-word is a marker of group membership. If you say the word, even when simply mentioning it, it says that you didn't get the memo about not using the word. Therefore, you might not be part of the anti-racist in-group. And if you aren't one of "us", then you're probably one of "them". And therefore maybe even just mentioning the word becomes morally akin to actually using it.
You can look at this as similar to other arbitrary-seeming abstinences that groups take up. It is almost a feature that the prohibition is arbitrary because it emphasizes that the abstainer is doing so for group signaling reasons and not for pragmatic ones.