AFAIK, the Unix filesystems of that era had a limit of only 14 bytes for the filename. With a limit that short, every character counts.
(Nowadays, the most common filename size limit seems to be either 255 bytes or 255 characters, depending on the filesystem.)
On Unix systems people rarely used file extensions like this. Any program that you would run simply had no extension. It’s the shebang line that would launch the correct interpreter. Extensions denoting the file type was more of a Windows thing so it knew what app to use to open the program.