Because it's a word originally used from white people to put down and dehumanize black people. White people saying nigga in casual contexts displays a lack of sensitivity towards the black race's history of being systemically oppressed.

Why do black people use the word, then? Because the word nigga has been reappropriated. The word has gone through a process of its meaning being redefined within black circles to represent the struggle black people have been through, both as a part of their ancestors' histories within America, as well as the present day. It's okay for black people to use the word within this context cause theres the assumption that this notion is mutually understood; this is not always guaranteed the case for white people

EDIT: more on the topic bc i feel strongly about this

People get mad at white folk who use and want to use the word nigga because white folk use the word because it's 'trendy'-- it's a word that a lot of 'cool' hip hop artists, athletes, and etc use. But in reality, the word is incredibly nuanced, and means far far more to black culture than a word that is trendy and fun to say. Lack of respect for the term can, in effect, reduce systemic oppression against blacks to something that is 'hip,' 'trendy,' that white people can take part in, despite having nothing to do with the struggle itself.

Hope I made sense, and hope u guys gathered something useful from this.