Final answer:
The main success/reason for the Pony Express was its ability to deliver mail quickly and reliably across long distances using a horseback relay system.
Step-by-step explanation:
The main success/reason for the Pony Express was its ability to deliver mail quickly and reliably across long distances. Before the telegraph and other modern forms of communication, the Pony Express used a horseback relay system to transport mail from Missouri to California in just 10 days. This was a significant improvement compared to the previous methods, which took months. The Pony Express played a crucial role in connecting the East and West coasts of the United States and facilitating faster communication during a time of rapid expansion and westward migration.