Where are those numbers from? 3% seems really low. I would guess the IE8 (being the highest version that can run on XP) population is much higher then that, but IE9 was chosen to make the number look better.
People should always run their own numbers. It can vary quite a bit from the internet-wide average.
If I had to guess, it's simply because upgrades are such a hassle in heavily regulated industries. (Above and beyond the typical hassles of an upgrade in any large enterprise.) I see similar numbers in Healthcare, for example, but not Education or Construction.
I keep meaning to do a blog post about the tech trends in different industries.