It was found that Six Sigma techniques provide great results when focus is on defect reduction. When it comes to productivity improvement or speed, Lean principles were found to be more effective.
Six Sigma certification was extremely popular till 1995. As Lean Management derived from Toyota Management System became more acceptable, integrating Lean and Six Sigma techniques made business sense for most companies.
Lean and Six Sigma were integrated, and integrated Lean Six Sigma is now the standard offering at most places.