Why was John the Baptist imprisoned?

Answer: He spoke against Herod's marriage

John was imprisoned because he told Herod it was not lawful for him to have Herodias, his brother's wife.

“Now Herod had arrested John and bound him and put him in prison on account of Herodias, his brother Philip’s wife, because John had been telling him, “It is not lawful for you to have her.””— Matthew 14:3-4 (BSB)