The city of Corinth was fully immersed in the Greek culture. The "golden-tongued" orator was the celebrity of that day - above athletes and other accomplished individuals. Public speech was regarded as the highest form of entertainment. The orator was always trying to win the audience with his way of words.
So when Paul came to Corinth, they were sizing him up based on his abilities, eloquence, and even his size/physique. By Paul's own admission, he was lacking in these regards. With regard to diction and sophistication, Paul simply did not meet their expectations.