In Windows operating systems the taskbars default position is at the bottom. Why is that so? Especially it is strange to me because all windows application have their main menu bar at the top. To me it makes more sense to have the main operating systemu menu (which the taskbar is) at the top where all other program have their main operating menu, where they are controlled from. Web browsers with tabbed interfaces also have tabs at the top, not the bottom.
To my knowledge it was not a design issue but a technical one in win 95.
The taskbar should be at the top, but many of the win 3.1 app use absolute positioning on screen. And the top left 0,0 used to be in application "space" in win 3.1. There was too many issues with a taskbar at the top. It was decided to put it at the bottom to lower bugs. Nowadays every app can work with taskbar anywhere.
edit : after some search I find a reliable source
Keeping it as far away from where applications have their menu makes a clear distinction to the user that each is for controlling something very different.
Applications are controlled by menus at the top, the system is controlled by menus at the bottom.
Mac has also experimented with this dual concept. For instance the application dock is at the bottom, and open apps are placed there. But the application specific behaviour (not minimising, closing) are kept in top menus.
But mainly, the reason the taskbar is at the bottom is artbitrary, it's been there since Microsoft first added it in Cairo. Now users expect it, so to keep a consistent and predictable interface it is adhered too. See more about menu conventions for windows apps here.
Partly arbitrary, but also historical. Mac OS didn't have a dock at the bottom until OS X and then it made sense to put it at the opposite of that standard menu bar, check out http://www.asktog.com/columns/022DesignedToGiveFitts.html to learn more about how there is a difference based on ergonomics of where you put things.
I know this is a bit of an old question, but here's a possible extra reason: It means that the cursor movement doesn't need to be precise when navigating an app. If the taskbar was at the top, you couldn't just throw the cursor to the top right corner to close a window, or the top left corner to access "File". Having everything in corners makes it just a tiny bit faster to use, because you don't need to "aim" for the main commands of the app.
After some digging I found a post where they explain, that most users (98%, old data, but probably still relevant) don't relocate their taskbar despite the settings being just a right-click away, which may imply either that 1) (despite the easy access) users are unaware of the setting or 2) perfectly content with its placement.
According to Raymond Chen, of Microsoft, one of the reason the taskbar ended up at the bottom of the screen is due to programming bugs by third-party applications. He explained the reason in this blog post at MSDN.
Before Windows 95, a lot of Windows applications assumed that they can place their windows at the coordinates (0,0), which is the top-left corner of the screen. (The correct way is for the application to query the OS first on what are the range of coordinates that is safe to place the window.) So, if the taskbar (which was introduced in Windows 95) was placed at the top of the screen, windows that are placed at the coordinate (0,0) will have the entire title bar (and part of the windows' content) falling underneath the taskbar. Not only that, but because the title bar is obscured, it is difficult / impossible for the user to move the window away using only the mouse.
Therefore, the taskbar was placed at the bottom of the screen to allow these pre-Windows 95 applications to continue to work.