To raise revenue for internal improvements.
Clay’s plan was to keep tariffs (taxes on imports) high. The money raised would then be used for internal improvements such as roads, bridges, and canals. All sections of the country would benefit, Clay claimed.
But the South didn’t industrialize. Their economy was based on cotton, tobacco, and slaves. So they had to import almost everything, and thus they payed more in tariffs than any other section.
The North had industrialized, so they had to import far less than the South.
The South also objected because most cities, factories and people lived in the North, so most of the roads, bridges, and canals were built there.