The majority of hypno content tends to be audio-only as most of the time you're being put into a trance and the tendency is for your eyes to close so the visual component isn't as vital.
That said, there is a relatively large amount of video based content out there but you can see (heh) the problem with video when you're likely going to have your eyes closed at some point.
That isn't to say that hypno/trance video based content isn't effective, for some folks that's they're preferred medium, it's just not the dominant format (at the moment anyway).
Also the file sizes are much larger and harder to share for some folks.
Audio simply tends to be the most efficient format as it uses the "imagination" codec to get the visual stuff done :-)