Perhaps it would be better to look at the economies of the various horse sports to find out why some are more popular and command a higher price for their mounts. What we’d find is…most of the top horse sports (under both English and Western tack) are fueled by huge $$$$$ inflo/outflow. Cash prizes, cash awards, high dollar awards, sponsorships, money, money, money.
That is what drives the popular horse sports, and the attending sales of their horses. Where prize monies are linked to rides, the prices of horses goes up, as does the popularity of the sport.
Endurance around the globe misses out on all that cash flow…except in the Middle East. In the US the AERC has very strict rules regarding cash prizes and awards, and many of the big rides haven’t advanced beyond the silver belt buckle stage. Most of the small rides won’t ever advance past the t-shirt award stage.
This simple homespun mindset tends to discourage big money from making any type of major inroad towards developing high dollar endurance horses, horse sales, and the attending livery that goes along with the “big name rider/trainer/breeder” market.
However, one has only to dip a toe in the super high dollar Hunter/Jumper market to realize that not every horse showing is a 6 figure horse. Many riders/trainers still look at the super cheap (often freebee) Thoroughbred market for failed racehorses that might have the talent to become spectacular, high dollar winners. They also look for failed dressage horses, or horses burned out of other disciplines, to see if they can find that diamond in the rough.
Dressage does the same - they scout out the failed hunter/jumpers, the odd-sized horses that are low cost because they aren’t “top of the line” in height, the not-quite-winning comformation horses, the back barns of the racetrack. Dressage people here are also famous here in going to Europe and getting suckered into buying the rejects over there for less than the super bucks, then spending years and years trying to develop those hard-headed cases into something worthwhile.
Looking for a “cheap” horse isn’t limited to Endurance by a country mile. Rags to Riches is a common theme in almost every discipline.
That said, recent rule changes in Endurance seem to be shifting away from the “any horse in your barn can win at the sport” towards the “you WILL need an Arabian if you want to top ten or better”. Will that open a bigger market for endurance bred horses? <shrug> I doubt it. I think Endurance competitors who are serious in racing are already looking hard to find the genetic “freaks” that have super low heart rates and super fast recoveries. Breeding for that isn’t easy, and there still is very little money in the venture. Arabian racing, show, and halter are the top cash cows - so the majority of breeders are going to stick with that program. Endurance will remain on the outside picking up the rejects.
Cheap horse or not, I would also argue that there are MANY serious Endurance riders, probably far more serious than other disciplines, simply because we HAVE to spend more time with our horses, practicing and training to develop a style of riding that can be comfortable on the natural trail over miles of shifting terrain. I would NEVER want to ride in a dressage seat, or a hunter/jumper seat, or a stock seat for 50 miles! Yes, each of those riding styles brings something to the table for a rider interested in those disciplines (I was deeply involved in the first two for many years) - but these styles were developed for the show ring and jumping fences and cutting cattle. They are too rigid to translate into riding long distances. A rider needs decent balance, yes, but they also have to find what seat/leg position works best for themselves, and their horses, when it comes to traveling 50 to 100 miles in one day.
Unfortunately, because the sport hasn’t anywhere near the numbers competing compared to other equestrian sports, the knowledge of conditioning and feeding an endurance horse, as opposed to a hunter/ eventer/racehorse, can often be hard to find, or only accomplished by trial and error. Even the vast majority of vets know squat about Endurance, and what makes an Endurance horse tick, or even what makes the sport tick. Yes, I agree that if you ride balanced and know how to condition and feed properly, you will have a much better chance at succeeding right from the start. It is just more difficult in this sport to find the right program, and what works for someone else’s horse might not work for yours.
By default, our sport has to be more trial and error. We have to find our way for each separate horse, because that trail out there is the only reliable teacher to show you if your choices are good and successful, or bad and defeating.
Regarding those who “just won’t give up on a clearly unsuitable horse that’s miserable and stressed and hates his job” - yes, I’ve seen this happen, but very few and far between. One or two were good friends, experience endurance riders trying to work with a new partner that just wasn’t going to cut it for one reason or another. Eventually - sometimes sadly because they really loved the horse - they finally give up and find a more suitable partner. I knew one that shed tears for months afterwards because her beloved endurance-bred Arabian that she purchased expressly to do the sport…couldn’t. You can push a horse 25 miles… but not 50, and certainly not 100 if they just don’t want, or can’t, do the work. When it ceases to be fun for either party, the endurance trail is a wonderful “leveler” that tends to hurry the decision along to either drop back a level, or change partners.
There is a lot to this sport that just can’t be translated to the other horse sports that take place in a confined area, or in a confined space. And to that I can honestly say “viva la difference”. 
