The opening of the Sao Paulo Temple in Brazil was a contributing factor in bringing this issue to a head. Who was going to be allowed to participate in the Temple ordinances in a region of the world where there is so much inter-racial mixed people/marriages??

Spencer W. Kimball, as Church President and Prophet,
had the courage to question the policy and to really
pray about it with an open mind. I believe he got an answer... And he had the courage to move ahead and Change the policy.

This is my theory: The US population, in the mid 1800's, was very politically involved in what lead up to the American Civil War(1861-1865). Somehow
those political pursuasions leaked into the Utah Territory and had its influence.

Joseph Smith was criticised by his critics for being Anti-Slavery. It appears that Joseph had no problem with giving the Priesthood to people of African background.

His sucessors in the Utah Territory apparently were influenced by the Politics of that time in American thought. (That people were different because of
orgins and skin color).

Spencer Kimball had the courage to question and to Change that thinking. God never changed. He was just watching.

Unfortunately, there are many white Mormons who are indeed Racist to Blacks today, it was only 1978 that the ban was over.
Many Mormons have been taught for many generations that Blacks are inferior and cursed with black skin so it hard to change that idea.
There is really no major outreach by the Mormon Church missionaries to convert African Americans to Mormonism or to work with the African American community in anyway.
That is why there is not much new converts when it comes to African Americans.