I think a few years ago most of the clams been imported were fairly large > 4". At this size they have much more zooxanthellae and also a larger tissue mass and could survive from the light along. However, now we are seeing 1-2" clams been imported which cannot get enough nutrition from the amount of zooxanthellae they have in a small amount of tissues.
Also the phytoplanktons are now available which weren't then. At the time I beleive Knop did recommending feeding yeast to clams, but maybe lack of a proper food source and already nutrient enriched tanks meant this didn't become a widespread recommendation ? Maybe a combination of higher nitrate levels in tanks meant they also had plentiful supply of nutrients to increase the zooxanthalle density.
Just my thoughts....