If you work with the K-12 system in the US, you'll discover that a lot of your customers are still happily chugging along with IE8.
Another driver: ipads and chromebooks are taking over.
I think it is necessary that projects keep focused and that means deciding what not to do.
This is my client base at $dayjob. As of last year, the last of them had finally let go of IE6. It would be nice to think our influence had something to do with it, but Microsoft had been trying to get them to ditch it for years, and they have a hell of a lot more pull than anyone else.