The invention of the microphone is responsible for that. Before the microphone, singers had to be loud to be heard over the instruments and audience. With the invention of the microphone, it was possible to sing in a much quieter voice and still get heard. Singers who took advantage of this to sing in a quieter and more intimate style was called "crooners". The style became very popular when radio became mainstream.

Of course a lot of people hated the style (as always when something new came along) and thought is effeminate and ridiculous compared to the manliness of a trained opera singer like Enrico Caruso.