Most everything was washed in hot water because legions of houswives had been told it gave the best cleaning performace, and provided a sanitary wash. What woman would risk putting the health of her family in jeopardy by washing in warm or *gasp*, cold water. *LOL*
Before enzyme detergents, when laundering with soap and or the new "detergents" hotter water temps did provide best cleaning, especially of oils, grime and muck. Some soaps like Fels, and detergents that were petrol based did a reasonably good job at cleaning in cold water, but in general soap and many detergents of the day not only did not clean well in cooler water, they did not disslove/rinse well either. This was also the era of hot wash and warm rinse, a setting you'd be hard pressed to find anywhere on today's washing machines.
It should be noted that up until around the late 1950's or so, many bed/table linens, shirts, undergarments (non dainties of course), and the like were all designed to be laundered in hot water, even jeans. Textiles were woven differently, and sized to allow shrinkage.