Does Notepad still randomly destroy line breaks and not let you undo more than once?
If so, those are two good reasons.
That aside, Notepad is bad because every single other text editor is probably better. Syntax highlighting, macros, plugins, ftp, language based autocomplete, etc. You can build projects directly from a lot of them with a couple of keystrokes.
What isn't wrong with using Notepad? If you use Notepad you are either a) too lazy to see what else is out there b) ignorant c) stubborn or d) don't care enough about your workflow to improve. There is literally no way you could form an argument that Notepad makes you more productive than any other popular editor.
