why we need two connections between the ftp server and the ftp client

why we need two connections between the ftp server and the ftp client

According to FTP protocol (rfc 959), when a ftp client connects to a ftp
server, a control connection should be established between the ftp client
and the ftp server. And when the ftp client sends a request of {LIST,
RETR, STOR, etc}, a data connection between the client and the server
should be established first, and ftp data will be transfered in the data
connection.
so, my question is: why we need the second connection -- the data
connection ? why not all the request, reponse and ftp data be transfered
in the control connection ?