why we can not do to creat folder by name of "con"?

Answer 1

Because its reserved by the file system?

You cant make a folder or file with any of these names:

CON, PRN, AUX, NUL, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, and LPT9

Answer 2

It's from the time of DOS. It is reserved name for Console (standard input and output device), which really means your monitor and keyboard.
For example, in DOS, you could type this command:
C:\> copy con text.txt
This command will copy whatever you type at your keyboard to the file named text.txt. This will go on until you press Ctrl-Z, which means end of file for DOS. Then you return to the C:\> prompt.
Also, it works the other way round.
C:\> copy text.txt con
This command will display the contents of file text.txt on monitor.
Similarly reserved names are PRN (for Printer), LPT (for Paralle port) and COM (serial port).