There are 2 major problems with the Triple Crown nowadays regarding the horse population (never mind all the problems with the human connections):
(1) Horses reminiscent of champion sprinter Housebuster are now the popular type of Derby favorite
(2) There are hardly any horses reminiscent of Unbridled (or Summer Squall or Pleasant Tap for that matter)
https://www.youtube.com/watch?v=QUQbYpOC2mk