Why did Karl Marx write the quote workers of the world unite?

Originally it was "Working men of all countries, unite!" It was the last line in The Communist Manifesto which both Marx and Engels wrote. The slogan was eventually modified into "Workers of the world, unite!" primarily because it just sounds better and there's no potential for people to interpret it in any sexist way. Marx and Engels wrote the line because The Communnist Manifesto is basically a call for a socialist revolution, the working class is inexpendable to this revolution.