Most successful WP horses these days are bred with the conformation and mentality of having a low headset and slow motions. These 2 attributes are accentuated by training to what you see in the WP show ring. I am with you 7ponies, I don't much care for it either but that is off topic. Some horses also just naturally have a very low headset and that is pretty popular in all western disciplines. I prefer to have my horses carry themselves pretty close to level with the withers but some horses are just bred to carry thier heads very high and others to carry theirs very low. I have a QH that regardless of the training, he is not comfortable carrying his head level or below level. He is built to carry it up so that's what he does. On the other hand, my brother has a 4 year old filly that he just started this summer (she has no WP breeding) that very seldom raises her head above her withers. We don't train for a particular headset and just prefer to let the horses have them where it is comfortable for them (excluding peanut roller and head-in-lap, we train those away right quick).
Anyway, sorry, off topic. When WP competitions first began, they were designed to show off how controlled a horse was at all 3 gaits, how comfortable they were to ride, and how responsive they were to cues (hence the droopy reins). As the decades passed, judges began to judge the horses who carried their heads lower and moved slower as first in the classes because they look more natural and comfortable and relaxed. So people began to breed and train specifically for the low headset and slow legs. Humans that we are, we perverted the natural moving horses into what is referred to as "peanut rollers" which were horses who carried their heads no more than a few inches off the ground and moved so slow it looked like they were on the verge of death (how anyone finds that attractive I'll never know). The WP industry is getting better and moving back toward more natural looking horses but there is still a long way to go. Sorry, letting my opinions leak out again. I guess the moral of my novel is that when it started, a lower head and slower gait = a more relaxed and comfortable looking horse so that is what they started breeding for.