I’m going to hazard a WAG that the trend truly started with color photography.
Up until, say, the 1950s, American culture was roughly divided up by country of origin. Most popular culture starts in the big cities, where, incidentally, most immigrants settled. There were Irish communities, Italian communities, Jewish communities, etc. etc. etc. Prior to the 1950s, people mostly lived their lives in the same general area where they were born. On top of that, it was very common for people of a particular ethnic group to encourage their children to marry a member of the same ethnic group. Marrying within your ethnic group was easy, because almost everybody you knew was part of your group.
The result was certain racial traits, like red hair, being perpetuated for a long time. So at the time color photography was invented, redheads accounted for a considerably larger percentage of the American population. So when people started taking color photos, there was a reasonably good chance of finding a redhead in any given random trio of people. Also, consider this: people were already familiar with seeing blonde and dark hair in black & white photos. In that environment, redheads would naturally become tempting subjects for users of newfangled color film. I can also easily see redheads being eager to pose for photos after years of being indistinguishable from blondes or brunettes in black & white photos.
Thus, the mere invention of color photography made including three hair colors Standard Practice™ and it has remained Standard Practice™ while actual hair colors have become diluted due to people started moving about the country and finding themselves with fewer options for marrying within their own ethnic groups. My mom once commented that “dishwater blonde” is now a particularly American hair color.
I’ve been lucky enough to have one redhead in each of the last four generations of my family, which is heavily English and Scottish. My maternal grandmother was a redhead, my dad’s sister is a redhead, I have a redheaded cousin, and now a redheaded niece. Probably because my brother-in-law is also of Scottish ancestry, my niece’s hair is more brilliantly “pure red” than that of my other red-haired relatives.
And I’ll agree that redheads are hot. I’m biased, though, because I lost my virginity to an Irish redhead, on St. Patrick’s Day.
Another BBR trio: female country-singing trio Shedaisy. And the redhead is the hottest one in the group.
