Then the butter is washed thoroughly to get out all remaining protein particles. Finally, the butter is kneaded to get out as much water as possible, salted and formed. Since man began to make and use butter, he made it from ripened matured cream - sour cream. A change to unsoured or sweet cream butter came only in the forties of this century. The reasons for the change were purely technical.
Machines work most economically and profitably when they run permanently.
Buttering machines were constructed that transformed sweet cream endlessly into butter. Sour cream at this time resisted this process. You had to fill the churn with one batch of sour cream, finish buttering, clean the churn and start again. Thus for purely technical reasons, people became used to sweet cream butter. The standard book about butter making from 1915, Principles and Practise of Butter Making by McKay and Larson, does not even mention sweet cream butter.