The poem, written by Julia Ward Howe, was mostly about slavery. Howe was upset because slavery still hasn't been abolished fully. The title, The Grapes of Wrath, was taken from that poem. The book wasn't about slavery but about rich owners using poor migrants has workers and not giving them enough money to feed themselves or their family, and not giving them the rights they need.
The biblical passage Revelation 14:18-20 also has something to do with the title. The book