The last Triple Crown win happened in 2015 when the horse American Pharoah, jockeyed by Victor Espinoza, won the Kentucky Derby, the Preakness and the Belmont Stakes. Only 12 horses have won the Triple Crown, the first being Sir Barton in 1919.
Twenty-three horses have won both the Kentucky Derby and the Preakness, but fell short in the third race at the Belmont Stakes. The track at Belmont is longer than at the other two venues: 1.5 miles long versus 1.25 at the Kentucky Derby and 1.1875 at the Preakness. The increased length presents an endurance challenge for which quicker horses on shorter tracks might not be conditioned. Also, due to the high humidity and frequent rain at Belmont Park, a much sandier surface is used for the track than at the other two tracks. This difference also might present a challenge to horses that do well on the other surfaces.
Also, with the Belmont Stakes being the third race in a five-week span, it may simply favor horses that have either not raced in one or both of the previous two, or favor horses that have not raced hard enough to win in the previous races.