How did the Kentucky Derby become such a big and important race?

The Kentucky Derby has run since 1872 making it one of the oldest stakes races in North America. (The Travers Stakes at Saratoga is the oldest, having begun in 1864.) A "Derby" race in North America is a race for three year old horses, mostly male horses. The Kentucky Derby is the best known derby both because of the long tradition and because it tests these very young horses aver a distance that they almost certainly have not run prior -- one mile and a quarter. The Kentucky Derby is also also has one of the largest purses of North American three year old races. The tradition and richness of the Kentucky Derby is enhanced by two other challenging races for three year olds: The Preakness Stakes and the Belmont Stakes. The races in combination are called the Triple Crown. The Belmont stakes is the oldest of the three, having started in 1866. But unlike the Kentucky Derby, the Belmont Stakes was not held at the same track over the years and for two years was suspended (by law) entirely.