I was very careful in the first post to say "When 6 Sigma doesn't work", not "Why 6 Sigma doesn't work".
It doesn't work when you measure the wrong thing. You don't measure the tool, you measure the output of the tool. And unless you are selling packaged software, then the software is the tool.
If someone says they're doing 6σ, and all their metrics are on the tool or process instead of on the product, they're not really doing 6σ.