Why do you think some modern poets still write in the highly structured sonnet form as well as in free verse?
As an opinionated question, there are many possible answers to this question. The form of the sonnet, including the rhyme and meter as well as the typical themes, represent both creativity and discipline. While modern literature tends to lean more towards free verse and defying tradition, the cadence and beauty of the sonnet is a form that will continue to connect to the audience. They provide a challenge for the author as well as depth to the audience. Additionally, every generation of artist interact with previous forms in some way, whether through imitation, evolution, or rebellion.