Mark wants to know why Windows has never supported having a taskbar on more than one monitor. (The question was asked before Windows 8 multi-monitor taskbar support became publically-known.)
The feature has always been on the list, but it's a long list, and specifically the cost of designing, implementing, testing, performing usability tests, then redesigning the feature (because you will definitely need to redesign something as significant as this at least once) historically prevented it from escaping the minus-100-point deficit.
Features do not exist in a vacuum, and decisions about features necessarily take into account the other features under consideration. For a feature to be adopted, it not only must be valuable enough in itself, but it almost must provide a better cost/benefit ratio than any other features under consideration. While the benefit of a multi-monitor taskbar is high, you have to scale it down by the percentage of users who would be able to take advantage of such a feature. I don't know the exact numbers, but I would hazard that fewer than ten percent of users use a multiple-monitor system on a regular basis, so any benefit would have to be ten times as great as the benefit of features that have broader use.
On top of that, the development cost of a multiple-monitor taskbar is significantly higher than most other taskbar features. Just the compatibility constraints alone make you shudder. (Think about all the programs that do a
FindWindow looking for the taskbar and assuming that there is only one.)
What changed in Windows 8 that made a multiple-monitor taskbar a feature worth implementing? I don't know, but I can guess. First of all, the overall engineering budget for the taskbar may have been raised, so that more features from the list can make the cut. Or maybe the people in charge of the taskbar decided to go with their gut and ignore the numbers, implementing a feature specifically targetting the enthusiast community even though the work would not be justified if you went strictly by the cost/benefit analysis. By doing this, they ended up short-changing other features which were perhaps more worthy if you looked at the numbers.
And then you'd be asking, "Why didn't you do feature Y? I mean, it would have been far more useful to far more people than the multiple-monitor taskbar."
(Of course, now that I mentioned Windows 8, everybody will treat this as open season to post their complaints here.)