Because it is the first race in the Triple Crown and if a horse wins it then there is a possibility for a Triple Crown winner.
The first horse to ever win the triple crown was Sir Barton. Sir Barton was born in 1916 at the breeding farm of Madden and Gooch in Kentucky.http://triple-crown-racing.suite101.com/article.cfm/sir_barton_won_first_triple_crown
Sir Barton
The Kentucky Derby is the first leg of the Triple Crown, a horse that wins the Kentucky Derby, The Preakness, and the Belmont Stakes becomes the Triple Crown Winner
Citation, the Triple Crown winner of that year. Citation was ranked number three of all time by "The Blood Horse' poll.
horse racing was probably the first sport to use the term triple crown. obviously, this term is used in other sports such as baseball, but it originated from horse racing.
Secretariat won the triple crown in 1973. He was the 9th horse to do so in 25 years.
Horse racing
Seattle Slew won the Triple Crown in 1977.
The horse that won the TC in 1919 was Sir Barton
Secretariat
14 Players Won the Triple CrownThere have been 14 different players that have won the Triple Crown in hitting with two players dong it twice. Roger Horsby in 1922, and 1925, and Ted Williams in 1942, and 1947. The last to hit for the Triple Crown was Carl Yastrzemski in 1967 when he hit 326, with 44 home runs, and had 121 RBIs.