Because they are purpose bred to be incredible athletics and naturally they will do very well in jumpers and dressage. Other horses can also succeed in those sports, but they are usually purpose bred for something else so it's not as likely - Ex. Thoroughbreds are bred for racing, not to be jumping horses, but some of them succeed anyway. Quarter horses are bred for ranch/western work, but some of them manage to be successful in the english disciplines.
With warmbloods, it's more of a sure thing that they will succeed, as that is what they are being bred to do.
Well, I guess because Warmbloods tend to be very muscular and handsome. I understand that warmbloods might be a type of horse that comes out on the internet when you search "muscular horse" or something like that but they are no better than any other horses. They also tend to be sometimes stubborn ( a warmblood I know is like that so please don't take this personally). I think that all horses are equal no matter their breed, personality, or any of their features. Let's fight for equal rights of horses guys! :)