Why upgrade any Windows version if your current one is still running smoothly and without any problems? I'm still working on W2K and I'm not planning to change that any time soon. XP has some nice features, but there is nothing that I couldn't live without.
For companies it's just the same. Especially when you have 3rd party software that is not (yet) optimized for XP it's a risk to change the OS if you don't need to.
And last but not least: updates don't come for free.