A more enlightening question would be: "Why do even good teachers often not make the difference?". Consistent factors act as blocks to learning: lack of parental support for education and concentrations of children who run wild out of school in certain class groupings, to name but two.
It would be valuable research that sought to identify which factors would be the most significant blocks to learning and in which combination. Undoubtedly, quality of teaching would sit near the top, but not in isolation.
However, this would cause a significant difficulty for the chief inspector, as it does not serve the political argument that he seeks to pursue. Causation is a complex web of factors and the simplistic answer - that good teachers would be able to overcome all of the many societal problems that block their pupils' education - does a disservice to those pupils and to many dedicated teachers.
Acting headteacher Tullyallan School Darwen, Lancashire