The decline of traditional colonialism after the two world wars meant that the days of large overseas empires was at an end. Self-determination movements around the world made the idea of continuing imperialism less savory. Powerful nations interested in exploiting the resources of weaker ones had to find a new system. They turned to what became known as neo-colonialism.
In order to maintain a status quo that kept them at the top of the global pecking order and continued to provide inexpensive resources from overseas, the former imperial powers devised ways to keep their former colonies weak. This included dividing them up into states that were designed to fail. Either they would draw up borders that left the country without access to much needed resources or leave it with a government incapable of effectively governing. This created conditions in which the former colonies needed to continue to rely on their former colonial powers for economic and military protection.
By creating a system in which one nation was forced to rely on another, even if informally, neo-colonialism was made possible. As long as strong countries still find it advantageous to exploit weaker ones, it will likely continue to occur.