Not just restricted to windows, alt+tab is also used in many other graphical desktop environments such as WindowMaker, CDE and KDE.
While Microsoft's windows has established this key combination as the defacto standard for application swapping, it makes sense for such a combination to be the alt key and the tab key pressed together; using the left thumb on the alt (or possibly meta key) and the ring finger or little finger on the tab.
Although that reason doesn't take into account left handed people or non-pc keyboards...