Why do the call a "soap opera" a soap opera?

 Profitbob is a bit younger than I and only knows of television programming. Soap operas began in the mid to late 1930's as daily serial entertainment for housewives broadcast on radio. It is true that most of the sponsors were soap companies. Each program was for 15 minutes and was based on sad, sad stories that continued for years. They were called operas because the plot of every opera is a story of tragedy that brings tears to the audience. In those day I too listened to some when I returned home from school at 3:00 PM. There was Ma Perkins, Stella Dallas among others. Sposors were Duz, Oxydol, etc.