It has much to do with the safety margin of Quest (Moxidectrin) vs the others. Moxidectrin only has a safety margin of 3 to 5 times overdose. When you compare that to others, the nearest is 20 times, and Ivemectrin is 60 times the amount before adverse reactions appear.
Now with that said, I will give you my opinions on Quest. I would NEVER use it on ANY horse. First, with minis, they seem to be much more sensative to it. I used it once when it first came out, and it caused my mare to abort. This was the only thing given to that mare, and she was preg checked by a vet the weekend before. It happened within 24 hours of giving it. Also, it was dosed at the correct dosage, so that was not a factor. And I was lucky, many minis have actually died from it.
Now why won't I give it to my full sized horses? Well, it has to do with the other animals, namely dogs, that are on the place. My dogs love to eat "horse apples". Well, when you deworm the horse, guess what goes out of the horse's body with the "horse apples"? Yep, the dewormer. And it was shown that many dogs react to Moxidectrin negetively. In fact, there was a product made by the manufactures of Quest called Pro-heart 6. The dewormer was supposed to be effective for 6 months at a time. It was made from Moxidectin. Guess what? Many dogs got very ill or even died and there was no way to get it out of their system. Further, I have heard of dogs dieing after a horse was dewormed with this because they ate the "horse apples".
I will not use a product that has so much questioning on the safety margin of it. Here is an article on dewormers and deworming program that talks about the safety margin of different products.
http://www.equinenet.org/ernet/worms.html
So in short, why would I use such a product when I can find other products that are just as effective yet have a higher safety margin when used in a good deworming program that consists of rotating different types of dewormers?