It is sort of a convergence of reasons.
The deeper you go in the ocean the more the light shifts toward blue. As such corals are adapted to absorb more in the blue spectrum with some taken in the red spectrum. Green is effectively wasted on them. Reef-keeping culture has a bit of an obsession with maximizing the usable light for their corals so they tend towards blue light with a peak in the red spectrum (which is often lost in the overpowering blue light). This has led to blue basically being the fashion of reef tanks. Just as 6700K is the fashion for planted tanks. It has become automatically assumed that reef lighting should tend toward blue, just as it is assumed that planted lighting should shift toward daylight. And it helps that corals flouresce in moon light, which tends heavily towards the blue. Meaning that many corals will look drastically better with heavy blue supplement (ie: actinic or royal blue LEDs).
A reef can run perfectly fine under daylight, just as a planted tank can run just fine under actinic. It may not run as efficient though, and the color may not be as nice, but it is perfectly doable.
