For me it’s because they are very careful not to screw over customers that rely on certain behavior. Having said that, Microsoft does sometimes break backwards

For me it’s because they are very careful not to screw over customers that rely on certain behavior. Having said that, Microsoft does sometimes break backwards compatibility. I sell a Windows app (a battery meter) that cannot work on Windows 11 because the new Taskbar doesn’t expose an interface for plugins to render data on the taskbar (IDeskBand is the old interface). So I can display a small window to the user but it’s far less convenient that showing it persistently in the taskbar.