Six sigma wasn't really a project management solution. It was an iterative process framework specific to manufacturing where the number of defects would be measured, the causes of common defects were identified, then resolved. The process would continue to iterations until the number of defective widgets produced was reduced below two-in-a-billion.

This remains a very effective approach in large scale manufacturing. Many engineers still learn about this in school; if not explicitly six sigma, then at least the statistical underpinnings and application of it.

Six sigma was a victim of its own success. Business leaders tried to expand and generalize this framework to apply to different areas of business. This doesn't work well because building a billion nearly identical widgets is nothing like managing a team of accountants.