It's legacy, white spaces weren't allowed as characters on most FTP software, which is how the warez scene shares it's releases. It used to be underscores, but dots are closer to a white space regarding separation (space wise), so most release groups use dots nowadays.
Generally, a white space as a character in filenames and directories is "frowned upon" in many operating systems, Windows included (somewhat). It makes writing scripts and software more comlicated because it's used as as a separator for giving command line/terminal options to commands and binaries (programs).
Yes, you're correct, it goes much further back than FTP, all the way down to UNIX I believe. The problem was commands and parameters (options) which use a white space to seperate between them. So, filenames and directories were't allowed to have white spaces in them.