Symbolism and scarcity.

As for symbolism, red is a powerful color that symbolizes power, passion, danger and bloodlust. There’s also a hidden, metaphysical/spiritual/archetypal symbolism: a darkened red is the first color we “perceive” as we’re babies inside the womb, as the red light can reach the human tissues deeper than other light waves. Speaking of light waves, red is the longest visible wavelength, which kinda of seamlessly blends with infrared, invisible to our eyes, but can be seen by some predatory animals such as snakes (which adds to this symbolism). We can also note how things are “redshifting” throughout the entire fabric of spacetime: as the universe expansion accelerates, light becomes more and more incapable of reaching far, thus leading to a redshifting of distant stars, galaxies, nebulas and other distant space stuff. Billions of years from now, the expansion is theorized to be so fast that individual atoms will break apart, an ultimate cosmic event that is called as “Big Rip”. If this were really to happen and there were still human beings alive, we’d start to see an ever-approaching redshifting, starting at the star we’re orbiting (because our current Sun would be already “dead” by then, so would be the Earth and the current solar system) until eventually a redshifting horizon and, ultimately, redshifting arms and hands as every biological form still alive would start dying. Phew! I digressed… But you got the point. “Redness” can be said as something intrinsically woven within the entire fabric of spacetime, so outside our control, so powerful than anything, so red is simultaneously power, beauty, passion and danger.

As for scarcity, IIRC, the redhairness is a characteristic caused by recessive genes. So it’s not as common as brown, dark or blonde hair, therefore, it can be seen as a symbol of uniqueness. Nobleness and royalty implies a sense of power and uniqueness, so it makes sense to depict queens and kings as redhaired.

Together, they sum inside our “collective unconsciousness” to see red (as well as purple, which symbolizes the other side of visible spectrum, blended with ultraviolet, also invisible to us but visible to certain animals) as a fundamental color for powerful figures.