Of course there are viable reasons.
It's not free is it?
Discounting repurchasing, retraining, admin, security and rollout, waht if you have in house apps that won't work post XP. Don't even mention compatibility mode... Then there's what do you move to Vista SP", Win7, wait for Win 8, a linux distro, how and when and where the money coming from.

Place before this one was still using Microvaxes and VMS 5.5 using code written in Fortran 77, sounds silly but the cost of moving on and (windows would be a highly dubious choice to move to in their environment was huge.
If for whatever reason the business puts the breaks on keeping up with the tech, catching up is not yet a viable proposition....
It will take impending doom to force that and a viable market, busines heads might say sod it and let it fold...