Since Affirmed nosed out Alydar to win the Triple Crown in 1978, ten horses have entered the Belmont a mile and a half from immortality. All failed. Why didn't any of these horses succeed in their attempts to capture the Triple Crown?
1. The dice went cold. As three year olds can improve markedly at a moment's notice, sometimes in a weak year, a 3yo can get good and parlay it into a pair of wins in the classics, only to flop in the final leg, usually due to breeding or fatigue from the racing schedule. This is the equine equivalent of getting on a good run at a craps table but having it end with a 7 before hitting the biggest bet. This happened to Pleasant Colony, Alysheba, Charismatic and War Emblem. Even after their Preakness wins, nobody thought any of these horses were juggernauts, and few were surprised that they lost the Belmont.
2. They were lucky to be where they were. Some horses do win the Derby and Preakness despite it being clear they weren't the best horse in either or both race. Easy Goer fans blame Pat Day for Sunday Silence's Derby win, and the Belmont romp was their vindication. Silver Charm, while talented, was lucky to win the Derby and was even luckier when Touch Gold had a historically bad trip in the Preakness. It surprised nobody when Touch Gold turned the tables in the Belmont. And either because of hoof issues or just having a bad day, Empire Maker lost to Funny Cide in the Derby for no apparent reason, but paid back the favor in the Belmont. In all 3 cases, it was hard to argue the better horse lost the Belmont.
3. Human error. Spectacular Bid was one of the 10 greatest horses ever and lost the Triple Crown because his jockey got him in a suicidal duel with a hopeless longshot. Stewart Elliot, similarly in over his head, ended up having Smarty Jones duel on the lead for the first 10 furlongs only to have nothing left to fend off grinder Birdstone. And Kent Desormeaux was roundly criticized for moving prematurely on Real Quiet, who was nosed at the wire by the late-surging Victory Gallop. All 3 of these horses, with slightly better rides, should have been Triple Crown winners.
We can't dismiss any of these scenarios for Big Brown. He may have simply been hot for 2 straight races and will now get caught come furlongs 11 and 12. Casino Drive may turn out to be his Easy Goer/Touch Gold and surpass him in the stretch. And Desormeaux may foul up again, opening the door for someone like Denis of Cork.
Or, he just might be that good and be our 12th Triple Crown winner.