Why is it so difficult for a horse to win the Triple Crown?

The Shape of the horse after the race is the most important, how well the horse did, is a factor, and if the owner feels it has a chance too.. sure there are others also..

S G/ 5/ 22

You don't train to win a triple crown .  You breed and condition to win a triple crown . We breed our Thoroughbreds with the hearts to run but no longer with the bodies to do so .
 
Our quest for speed has been at the expense of the Thoroughbred horse .  Winning a triple crown starts in the breeding shed .
To many are breeding a refined down Thoroughbred trying to obtain more speed . In this quest for speed we have lost , bone structure and the conformation that is needed for 3 year olds to race and remain sound . 
 
 This is proven again and again .  We have horses today that can't take the pounding of the " Big Three "  , so their owners and trainers will run them in 1 or 2 of the Triple Crown races .  The ones that run in all three are exhausted because of their conformation in many cases . 
 

Apparently we did not learn a thing from the tragic death of Barbaro and Eight Bells to name two .  Eight Belles ran with the heart of a champion , breaking her cannon and sesamoid bones in both front legs and was euthanized on the track as America watched in horror . Many in the Thoroughbred industry warned and continue to warn us about these conformation flaws . We breed these horses with the heart to run but in to many cases not the bodies to do so .  Then we lead them off the track with all their conformation flaws and in some cases dragging a leg behind ( Big Brown ) so we can send them off to the stud to Sire more of the same  . 
 
The Thoroughbred racing industry is well aware of their self induced image problems , starting with the number of Thoroughbred's that are sent to slaughter simply because they cannot run fast enough .  Not to mention the injuries on the track which in many peoples opinion starts in the breeding shed . The good news is more and more in the Thoroughbred racing industry are attempting to correct these conformation flaws . It's going to take some time since we didn't get to where we are today over night . 
 
The Thoroughbred Racing industry and more importantly the Thoroughbred as a breed of horse could certainly use a Triple Crown Winner .  And many in the industry are working very hard to see to it that the Thoroughbred returns to it's glory days .  By breeding horses like we use to such as Seattle Slew , John Henry , Man Of War and Secretariat.   It can be done the great Curlin has proved that .

They are running against the best in crowded fields where anything can happen and there isn't much time for rest in between. When they get to Belmont they're up against a mile and a quarter exhausting distance which most have never before navigated. The Derby winner ran the best Beyer speed figure of his life and perhaps, too much higher than what he had ever done before. Horses that do that generally tend to come up a bit short in their next out as he did.