I guess the water was always pretty alkaline in some places, so soap was never an ideal cleaner. Around the beginning of the twentieth century, household detergents became available. Detergents have no problem with alkaline water, so they were really popular. They soon became cheaper than soap, too. So that's why people stopped using soap for almost everything, and started using detergents.
Detergents are excellent cleaners for most uses.
If you've tried using soap to wash your hair, you probably know how your hair goes all rough and tangly. I moved recently, and suddenly started having problems using soap in my hair. The problem is that hair reacts to acids and alkalis. Each hair has little scales, like scales on a snake or shingles on a roof. Acid makes the scales lie down flat, which makes your hair shiny and smooth.