Why?

Jeremiah’s intimate prayer dialogues were not canonized merely in order to preserve the prophet’s personal prayer life. If Jeremiah’s prayers were ever (auto)biographical, they are no longer only about him. In the canonical process they have become Scripture through which the reader can hear the voice of God. Jeremiah’s prayers were canonized because these human-divine dialogues have paradigmatic character and became important means for the instruction and edification of subsequent generations.—Standing in the Breach, page 334