Companies don't want to invest in a unstable and corrupt country. Even so, the rapid rise in HDI of the DRC aligns with the growth of their exports.
You seem to be implying that colonization is synonymous with globalization, but let me dispel this notion. The problematic part of colonization were the protectionist aspects of it. The Congolese were unable to compete for the same administrative positions of that the Belgians enjoyed.
Now you tell me how unions in the US are supposed to help the DRC.