Mr. Joseph Rosenberger brought shatnes awareness to American Jewry. Prior to his heroic efforts, American Jewry was largely ignorant of the prohibition.
When you hear all the nostalgia expressed by some for the time when American Jews didn’t have mechitzas at chasunas (and other various issues where Yiddishkeit was practiced with defects), remember the prewar American Orthodox Hamon Hoam were largely ignorant even as late as the 1950s and 1960s of large elements of proper Judaism. It was only after the postwar European Jewish influx into America when things started really to change. (There was some European rabbonim who arrived in the interwar period that also started getting things fixed in America.)