The last American Triple Crown winner was Affirmed in 1978. Since then, twelve colts have won both the Kentucky Derby and the Preakness but faltered in the Belmont, the latest being California Chrome. The long absence of a Triple Crown champion has been attributed to causes ranging from genetics and training regimens to the requirement that a horse run three grueling races over a five-week period.
While these may be contributing factors, a more obvious reason is simply that the amount of competition in the Belmont has been much greater since 1978. The average field size for the eleven Triple Crown winners was 5.4 with a median field size of 5. Sir Barton in 1919 and Count Fleet in 1943 faced only two rivals. Citation in 1948 and Seattle Slew in 1974 were in the largest fields, with seven other horses.
In comparison, the average field size for the eleven colts that came up short in the Belmont post 1978 was 9.5 with a median field size also of 9.5 horses. Charismatic in 1999 had the most competitors, with a 12-horse field, whereas Funny Cide in 2002 was in the smallest field at 6. Even the formidable duo of Easy Goer and Sunday Silence in 1989 did not scare off many competitors, as the Belmont drew ten entries.
Colts that won the Triple Crown from 1919 through 1978 raced in fields that, on average, were 43.2% smaller than those faced by the twelve colts since 1978 who won the first two legs of the Triple Crown but failed to win the Belmont.
Larger field sizes mean that the winner of the Kentucky Derby and the Preakness must contend with more fresh horses that have not been through the rigors of the first two races. Poor racing luck also becomes more of a risk as field size increases.
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Even the great Secretariat had only 4 horses running against him and Sham was spent by then. The rest of the field was pathetic.