Most businesses don't exist for the purposes of running and upgrading Microsoft software. Business preference for XP in general is because a lot of investment has been expended in establishing those windows environments. Migration to any different OS has a lot of cost associated and those costs need offer substantial benefits to be worthwhile. At a desktop level what Vista presents is essentially a sideways or back step.
Moreover MS has been hell bent on thrusting Vista upon the marketplace, and demanding the upgrade cost be absorbed. It is this MS arrogance, the forced redundancy, the massive over pricing, the refusal to listen to customers, and the extortive licensing that has end users considering alternatives to Windows OS.
The fanboys can get all exited about ribbon interfaces, some new pop-up annoyance, or just any change at all as motivation to incur huge hardware and software and time expenses of an upgrade cycle but in the real world PCs are tools for communication, for producing documents, running spreadsheet, building databases, and all the guff added to successive windows releases has done little to improve how PC achieve these tasks.
There is no particular love for XP and most have caught on that it offered very little over W2K and a move to XP created considerable pain for whatever upgrade gain it delivered. It is only those that use their PCs as toys and not tools that see another forced migration (Vista) as worthwhile.
MS have held a majority market share with Windows and with DOS before it for two decades, that was due to them offering the best cost/feature/ease of use compromise to the largest volume of users. The competitors have all improved their offerings considerable while Windows upgrades (well MS see them as upgrades) are becoming less for more with each new iteration.
Most users are not fleeing windows for other operating systems they are being pushed that way! A favour for XP represents the inertia, that has in reality delivered most users remaining comfortable with a windows environment despite its many flaws. But ignore or cost affect that comfortable user base, lock them out of updates, or force them into an unwanted upgrade cycle in a direction they do not wish to travel and then other OSs have become very real alternatives for many who would not have contemplated a move.
