Why? I guess because tar is black. We read:
"Leigh Lenzmeier stopped short of apologizing for referring to the St. Cloud Human Rights Commission as a "tar baby," but he did promise not to use the term again... He said there has been discussion for years about expanding the city's commission to include Stearns County. "When did we first hear this? I think, oh, nearly (back) to the ... (former Mayor Al Loehr) administration in the city, they've been wanting us to take over, sorry, this tar baby for a while. I don't see that," Lenzmeier said"..
Source
A "tar baby" is a sticky problem, that's all as far as I know -- and that's clearly what the speaker meant -- but the NAACP and various busybodies say the expression is a racial slur. Since careful people now have to speak of the pot calling the kettle Afro-American, I guess it figures.