How can that work for one breed, but not the other...
You can't tell me this QH is built like a Morgan or Arab... it's the dressage headset and collection
VS the western pleasure look.
BOTH are AQHA's
WHY IS IT DIFFERENT FOR A MORGAN???
TWO different looks for TWO different disciplines, but the SAME breed.
I just don't understand
Might be the same breed, but, going by the videos, two very different representatives. It's hard to say without a confo shot or work at liberty, but I would say that the "dancing quarter horse" is much more of the old-style cowhorse build, with a slightly higher neck tie-in, shorter legs, and all around more compact frame.
There is some form following function going on as well. Dressage asks for a different degree of collection than WP, no matter what the breed. That difference in degree means that muscles will be conditioned differently, resulting in a different "look" in the horse. Even so, if the second horse were conditioned as a dressage horse, he would still probably have a lower head/neck carriage than the blue roan.
By contrast, the pleasure horse is a much different type of quarter horse - leggier, longer bodied, with a lower neck tie-in. Different types, different bloodlines and breeding, two horses intended for two different functions.
There are different types of Morgans, too, usually characterized as "old" and "new" types. The old type is more of an all-around build, suitable for just about anything discipline-wise. The new type typically looks more like a Saddlebred or other gaited/park type breed, and is bred to excel in saddle seat classes. However, both of these types still have that higher natural carriage, and trying to bring it down to that level topline that comes naturally to the second QH is only going to be uncomfortable for the Morgan or the Arab.
It isn't even a breed-by-breed issue: it's a horse-by-horse issue. Any horse, properly collected from the haunches forward, will carry himself in the way that allows his individual conformation to bear the weight of the rider and execute the movement asked for with the degree of collection called for by the rider. What the individual silhouette looks like depends on the individual animal's conformation. Two differently-shaped horses in the same degree of collection will look different, not the same.