Answer: Because she had not given birth to a boy
Explanation: he had his eyes on one of his first wife lady in wating and she told him she can't be a mistress, so he decided to divorce his first wife with the claims that she didn't give him a male child to carry on his legacy. After the successful divorce he got married to the second whom by then was heavily pregnant but she gave birth to a girl instead of a boy. She was later executed on the claims of conspiracy against the king and cheating.
He got married to his third wife who was also a lady in waiting to the second wife. She had a boy for him at last.