Because there is a difference between 'fear' and 'dig in my heels and starve because of that fear'. Someone may fear elevators, but when pressed, I'm sure still begrudgingly get on because climbing 20 flights of stairs is more disruptive than their fear. They don't stop fearing elevators... they just 'get by'.
For probably 95 out of 100 that think they would never ride this... will be fine after they try. For the 5 that refuse after that... go deal with your irrational fear on your terms. The world isn't going to bend to you.
I think it's less than you might think. The only way to know for sure is a scientific survey.
Or, to notice that people going to Disney generally don't have an issue with the monorail which is as high off the ground as the gondolas will be.
FWIW there is no way in hell I will get on a gondola, but I will ride a monorail. I did it once, never again. Yes I fly in planes, and I will get on most of the Disney roller coasters (but have a hard time on boring standard coasters that are just a thin beam on a tall pole). The monorail is also fine. The truth is everything triggers my fear of heights, but hanging from a chord really tweaks it. Funny enough my stepdad will get on anything tall, and ride any ride, but wont get on an airplane. That's just how it works. You can go on and on about how safe it is, and yeah sorry doesn't help. It's also possible to hang someone upside-down from a 10 story rooftop safely - if done properly
Disney did send out a survey recently and I did tell them no way I am adding points on or staying at Riviera for that reason. Even if I could just get over it, the whole point of the trip is to relax, so no thanks.