As far back as 1947, when phone numbers only had 5 digits, a King-sized mattress meant a mattress measuring 6’ x 7’ in Southern California. Larger homes in California where homes tended to have more land, may have fueled the popularity of the bigger beds. It was called a California King to differentiate it from the Eastern King.
Another important factor was better nutrition. Better nutrition led to taller men and women and, eventually, wider men and women who wouldn’t be comfortable in a bed that only provided the width a crib mattress provides to an infant which is what a full-size affords two sleepers.
Outside California, the King size, which is the width of two Twin XL mattresses was embraced. Older homes in the east often had narrow stairways and a split king box spring could fit.
Now that your sheets, blankets, and mattress are more likely to be ordered online, the California King’s popularity is common across the country. The width of a King is more than most couples need.