I'm in the process of taking my Security+ exam, and I was taught by the professor teaching me that, when trying to figure out what port to put a protocol on (as there are a LOT of ports), if the protocol is combined with SSH or SSL, then the port number is by default 22 and 443 respectively. Fair enough, easy to do.

Now I'm studying and I see FTPS, which I interpreted to mean File Transfer Protocol combined with Secure Socket Layer (FTP + SSL), correct? So why then isn't on 443?

I did a bit of research on it, and it appears that there are several ports that need to be opened in order to maintain compatibility or something (according to https://en.wikipedia.org/wiki/FTPS#Methods_of_invoking_security), but even then, the ports are 990 and 989.

I can understand if one of the ports is 443 while the other is on another port, but I don't understand why NONE of it is on 443. Why is that?

Is it because of the older FTP clients not being compatible? The above link mentioned about compatibility, but I wanted to make sure that was the case.