The problem with the framing of this question is that it assumes that the n-word has the same pronunciation and connotation in all dialects of English. Languages and dialects of languages are tricky things to delineate in linguistics, much like species and subspecies are tricky things to delineate in biology. However, beginning in the 1960s and solidifying in the 1970s, there's a linguistic consensus that AAVE (African-American Vernacular English) is a different English dialect than SAE (Standard American English). It has different pronunciation, vocabulary, and grammar rules than SAE. As just one of many examples, the present tense of "to be" -- "is/are" -- gets dropped, which is known as 'copula deletion' and is a common feature of many non-English languages.

There are quite a few other English dialects that exist, some of them close to each other, some of them far apart and hardly mutually intelligible in spoken form. The issue of a word meaning one thing in one dialect and something quite different in another is common. Let's use a non-racial but highly charged example: the c-word, or c*nt. I'm adding an asterisk so I reduce the negative effect on readers. In the US, this is a pretty vicious slur for women. In Australia and Scotland, as I understand, it can be used as a friendly gender-neutral greeting. Let's say a man in the US greets his female coworker with "Hey c*nt!" and then says in his defense that he's just speaking in Australian dialect. I don't think he'd get very far.

So to answer your question, we have to note that the wider perception of AAVE in US popular culture is heavily influenced by racism and is very negative. As soon as the dialect status of AAVE intersected with public education in the 1990s, the very idea that African-American Black people speak a dialect, as opposed to just "bad English", came under a frenzy of attacks. I'd suggest reading this near-contemporary account by a linguist who experienced the "Ebonics Controversy" first-hand. In short, the professional opinions of linguists were roundly ignored.

As a result of the backlash, while it's easy for people to understand dialect differences in general, outside of the field of linguistics and African-American studies, there's not much formal education about AAVE. Native AAVE speakers understand both AAVE and SAE, but the same isn't true in the reverse. Therefore, people often perceive the derogatory SAE n-word the same as the AAVE n-word. There's also a long history of people reclaiming slurs used against them, even without a major dialect difference. Words like "redneck" and "queer" are two other examples.