There are two issues here:
1. The task bar, application, and process metaphor in Windows is not conducive to drag-and-drop onto taskbar items. Dropping onto an exe would start a process and pass the file as an argument, but doing this onto a window doesn't make sense. Does it start a new process from the file the window's existing process is based on? This would rely on that program recognizing it's already running and sending a message the other instance to open the dropped file then exit. If a program isn't document-oriented, this could be even harder to tell what to do. Theoretically windows could send a drop message to the process passing the window handle...
2. I vaguely recall a patent issue in the 2000s around this issue. At the time, lots of drag-and-drop actions resulted in explicit error messages indicating Microsoft knew what the user wanted to do but that it wasn't able to respond as the user intended. I quickly searched for the patent but could not find it.