Why you cant name a folder "CON" in Microsoft Windows?

It likely goes back to the fact that Windows is based on the DOS platform, and the (dot)con file extension is a restricted file extension for a DOS device file.

Con is short for CONSOLE and was used in early DOS days. We used it to create text files when we didn't have notepad or edit or edlin. It's a reserved word, like AUX (auxilliary ports) and PRN (printer). Could you use KON, CAHN, KAHN, or CONN? Hope this answers your question.