Why did the people of Lystra call Paul Hermes (KJV: Mercurius)?

Answer: He was the chief speaker

They called Barnabas Zeus and Paul Hermes "because he was the chief speaker" (KJV: Jupiter and Mercurius).

“Barnabas they called Zeus, and Paul they called Hermes, because he was the chief speaker.”— Acts 14:12 (BSB)