Why did God include Lamentations in the Bible?

The issue of canonicity in The Bible (i.e. whether a sacred book becomes part of the Bible or is excluded) is not divinely determined, so the answer as to why or why not a certain book is in the Bible does not come to God, but to man.

Normally, Lamentations is part of the canon as it was authored (traditionally) by Jeremiah who is considered a well-within-canon prophet. Its writing was also (traditionally) contemporary with other works such as Ezekiel and Daniel during the Babylonian Captivity.