The Manufacturing Belt was called the Rust Belt in the latter
decades of the 20th century because the word that describes the
deterioration of iron into rust was an appropriate name to give to
a region where the iron and steel and related industries were in
great decline. The South, West, and Southwest were called the Sun
Belt because of their sunny climates; Rust Belt mimicked that name.
In many cases, old factories had literally turned to rust, but the
whole region was considered obsolete and deteriorated.