They used to be transmitted over a hidden portion of the screen, during the NTSC and PAL era. That was topologically on the signal stream area corresponding to right under the video frame. Thus they were titles (text) that were under (sub) the image. They were also unavailable (closed) to the user until they activated them, when the decoder started drawing them over the frame by folding the signal so the text could appear. They originally were invented for and proposed to aid hearing impaired people by capturing the sound events, including speech, with short descriptive texts (captioning) .
That’s why in the US Closed Captioning is for hard hearing, and subtitling usually means only dialogue and other languages. While the rest of the world only uses variations of the word subtitling because English rapes and coopts everything it colonizes.