What it's really about:
It wasn't about sanitation or meat safety. Sinclair was actually trying to expose the exploitation of American factory workers and convert Americans to socialism.
He went undercover for several weeks as a meat packer and not only saw that working conditions in meat-packing factories at the time were horribly unsafe, but that there was massive corruption within the upper levels of management. The stockyards exploited not only the common man, but also the common women and children, who worked the same lengthy shifts and lost the same useful appendages to machinery without proper safeguards. At one point in the book, an employee accidentally falls inside a giant meat grinder and is later sold as lard.
A pinch of Mitch in every bite.
But much to Sinclair's frustration, the public's reaction was less "that poor exploited worker!" and more "HOLY SHIT THERE MIGHT BE PEOPLE IN MY LARD." They read right past the hardship of the workers and focused entirely on how gross the meat-packing process was.