Like gladiatorial contests, horse and chariot races were originally religious in nature. The chariot race apparently originated in ancient Greece. According to the Oxford Classical Dictionary (OCD, 3rd ed. revised), "in the funeral games for Patroclus [described by Homer] the chariot race is the premier event. The heroes drive two-horse chariots normally used in battle over an improvised cross-country course, round a distant mark and home again. Similar funeral games for other heroes are recorded, and heroes as well as gods were remembered at the Panhellenic festivals."
According to Wikipedia, "[t]he Romans probably borrowed chariot racing [and the race tracks] from the Etruscans ... who themselves borrowed it from the Greeks, but the Romans were also influenced directly by the Greeks." The races eventually developed into a fantastically popular sport and circuses -- chariot-racing arenas -- were built all across Italy and "in many parts of the empire," according to the OCD. "In the east, those of Antioch and Alexandria were famous, while Spain provides notable examples such as Merida (Emerita Augusta) and Urso, the latter famous for its racing stables."
The Circus Maximus was probably the most important arena of any kind in Rome, even after the construction of the Colosseum. Chariot racing was apparently even more popular than the gladiatorial shows and the races drew huge numbers of fanatical fans.
"The drivers were the sports superstars of their day," according to UNRV. "They could win greater name recognition across the Empire than even gladiators. One celebrity driver was known as Scorpus. He supposedly won over 2000 races before being killed in a collision ... at about 27 years old. Gaius Appuleius Diocles had perhaps the most recognition of all. He was a quadriga charioteer who drove in over 4200 races. His record included 1437 second place finishes and 1462 wins."
According to the University of Chicago, "[c]harioteers could win fabulous wealth, at least those who were freedmen, who always could threaten to drive for another faction. Prize money ranged from fifteen to thirty thousand sesterces to as much as sixty thousand for a single victory. Juvenal complains that a chariot driver could earn a hundred times the fee of a lawyer ... and Martial writes of Scorpus winning fifteen bags of gold in an hour. Diocles, a charioteer from Lusitania who competed during the reign of Hadrian and Antoninus Pius, won prize money totaling 35,863,120 sesterces before retiring at age forty-two."