The Soap Opera first came to be in the early days of radio. They were origionaly called 'Melodramas', or 'Operae Dramatique'. The "soap" part came in the in the 1930's, when corporate sponsorships began happening; Sponsored by, among other things, companies that made Washing-Up Detergent. The adverts became so common during these programmes they took on the name 'Soap Opera', as we know them today.
And Now you know.