Yes this is the reason, but the message-based part is irrelevant. They need to use an existing http connection where both sides agree on the upgrade to prevent lots of possible security holes.

If you aren't aware, the FTP protocol lets one specify a port and address to connect to; it was not that long ago that servers would not check if the address for the data connection was the same as the address for the control connection, and so one could send data to any port on the internet from an anonymous FTP server. This caused all sorts of headaches.