Oh, if you mean Jesse Jackson (?), the answer is: because you're not black.And sam,
Why is it ok for Jessie to call obamma "an uppity N*" but not myself?
When white people use the word, there's blatant racism in it. Even if you dn't mean it (like, if you grew up in the black projects, you likely will have black friends and use the N word like they do). But because others who don't know you will assume that by your use of the N word you are a racist, they either will be offended or wahtever, OR they are racist and they will think "yup, another N hater just like me, right on!" and that's not a message you should want to send.
And c'mon. It's the N word. If you want ot use it so bad, either use it in south Chicago around black dudes, or don't use it at all.