I'm sure that it is the US had abundant natural resources.
People were concerned about what groups were influencing politicians through their contributions.
Tim Keller on Dr. King’s rejection of relativism:
When Martin Luther King Jr. confronted racism in the white church in the South, he did not call on Southern churches to become more secular. Read his sermons and “Letter from the Birmingham Jail” and see how he argued. He invoked God’s moral law and the Scripture. He called white Christians to be more true to their own beliefs and to realize what the Bible really teaches. He did not say, “Truth is relative and everyone is free to determine what is right or wrong for them.” If everything is relative, there would have been no incentive for white people in the south to give up their power. Rather, Dr. King invoked the prophet Amos, who said, “Let justice roll down like waters, and righteousness as a mighty stream.” The greatest champion of justice in our era knew the antidote to racism was not less Christianity, but a deeper and truer Christianity.
(Reason for God, pp.64-65)
The united states eventually stop supporting South Africa because : South Africa began taking arms and funds from the soviets.
Back then, United States was in the middle of cold war with the soviets union, and this showed that Soviet has implemented its influence in south africa
Hope this helps
