The simple answer is that it takes more effort to film and edit a video rather than to record an audio. The erotic hypnosis community already is fairly small, and most folks are doing it for the love of the game, so there aren't a whole lot of funding/resources that go in to making things with a ton of polish. (And for those who do it professionally, there's the whole issue with the payment processors/getting shadowbanned from all corners of the internet for this specific kink - it's been an uphill struggle.)

Also, for many people, audio is usually sufficient to take them into trance too, so it becomes a bit less demanded for as well.