I would guess the problem for some adult beginners is that they are taught by relatively new instructors who only know one way to teach. If that approach doesn't work, then a beginner can think there is no point for any more lessons. They either quit or figure out ways to get down easy slopes that mean they quickly develop bad habits. ....
There are other things besides an instructor with limited experience that can cause a first day beginner to have difficulty getting themselves to turn left and right and stop. My comments below apply to group beginner lessons that do not last all day, not private beginner lessons.
1. The lesson is too short (1.5 to 2 hours). The beginner group is too large (12+ people). Either of these or both will mean there's not enough time and not enough individual attention by the instructor to help students individually if they are having difficulties that could be solved with just a bit more attention.
2. The beginner learning terrain is awful. This problem can take many forms.
--There's no carpet to take beginners up the hill for their first runs, so they have to side-step up to ski down. Walking up, side-stepping up, or herring-boning up, over and over, is exhausting. When tired, people fall more frequently. Then they have to get up, which is physically demanding. In a beginner group it can be demoralizing to get stuck on the ground unable to get up. Eventually tired students start falling more and more as their minds get fuzzy. They can't learn anything at this point and get terminally discouraged.
--There are skiers zooming through the beginner groups to get back to the lift (yes, I've taught on such terrain). This is legitimately dangerous and the adult beginners realize this. They are frightened. When feeling threatened, a student becomes defensive, which strongly blocks learning.
--There's no flat run-out at the bottom of the first-turns area. If the first attempt at stopping fails, the student has no alternative but to crash to stop. Fear freezes them up and they will never return.
3. The rental boots are two sizes too big. The student finds that their skis have a mind of their own. There is little control over where the skis point and no control over the edge angle. The student mistakenly thinks they are not naturally inclined to ski. They don't know it's the boots. They are doomed to not be able to hold a wedge; the skis wobble uncontrollably. If they can't hold a wedge, they can't turn left and right, and they can't stop. They think the failure of the lesson to teach them to turn and stop is because they personally are a failure. They won't come back.
4. The student is bow-legged. Or knock-kneed. Even if the rental boots fit well, the student cannot control the edge angle of both skis. It will be impossible for the bowlegged student to stop in a wedge; it will be impossible for the knock-kneed student to flatten the skis. Both situations leave the student feeling incapable of skiing. There is a liability issue with instructors stuffing folded trail maps inside the boot cuff of such students, and I've taught in ski schools which forbid doing this, even though it will temporarily solve the problem.
5. It's zero degrees Fahrenheit outside. The clueless beginner has no goggles, inadequate knitted gloves, no helmet, and no neck covering. These poor people are too cold to think. Being in danger of frost bite does not lead to eager learners, and their minds go blank. They wonder why anyone would like skiing, and can't wait to get into the lodge and warm up. They won't come back unless they have friends who ski who convince them that good ski clothing can solve the issue of being too cold. But if their budget prohibits buying all the stuff needed, they probably are lost to skiing.
4. The rental boots are too tight. The pain is excruciating. A student with this problem can't think of anything else. Their minds wander down to their feet, they can't attend to what the instructor is teaching. They reduce their movements to minimize the pain from the boots pressing against their bony feet, and miss what is being taught. All they want to do is get inside and take the boots off. They may persist with an exchange for a larger pair of boots. I hope so.
5. There are many other reasons to list that aren't about the instructor's experience. Don't always blame the instructor.