It's because of the incomplete research that's show how colour up corals by spectrum and they called it's the PUR, many lighting companies take advatage of it, especially the LED companies.
But even in the research for PUR, it's lacking of many essential thing: they don't show up the impact of the radiation from
UV-C, B, A to 380nm and ultra red to the growth and colour of the zooxanthellae, only doing research the impact of viewable radiation,
AND, the research didn't say that the reef needs only these PUR, but people, especially the hobbyst and manufatures thought that PUR is enough for reefing. Inside the reef, it's also many other photosynthesis creatures, like algae, bacteria, etc..., and how many affected by UV, Ultra red, green and red light also,
they make a trend to imitate the water quality like in the sea, but never imitate lighting like what is in the sea...not good for selling corals, many rainbow corals are brown under white light.
But now the reef industy is too big ,who can control the media and created the trend,
The more your corals are weak and died, the more you have to re-buy and fill out the empty space, the more coral shop be happy. The game is not easy like before.