See, before that, labor laws were thought wholly unconstitutional, as they interfered with the free market and all that. People started to accept them only after a bunch of paternalistic buttwads argued that we needed to protect all these feeble, lost women who had somehow stumbled into the workforce. Many started to worry about what would become of the poor dears who were fleeing the nest with no idea how incompetent they were. What if they couldn't compete with the boys and resorted to becoming, as Frank Reynolds calls them, who-ers? They couldn't make up for low wages with long hours because, as one judge argued in a 1908 case, women were simply too weak to work more than 10 hours a day. (Of course, after working 10 hours a day for table scraps, everyone would start to consider prostitution.)
So when these labor laws were struck down in 1924, it was actually regarded as a win for women's equality. A few years later, though, a little thing called the Great Depression happened, and people started to wonder if they might have taken this whole thing in the wrong direction. In 1938, the Fair Labor Standards Act (spearheaded by Frances Perkins, the first woman appointed to the U.S. Cabinet and a feminist icon if there ever was one) established the minimum wage as it exists today -- for people with all types of genitals, this time.
U.S. Dept. of Labor
"You're welcome, dicks."