Bees need some diversity in their diets. They consume nectar and pollen true, but that doesn't go far enough. If I offered you a diet of only Sugar cubes, Soya protein extract and Olive Oil you'd quickly die. Why? That diet has Carbs, Protein and Fat. That's everything a person needs!

It's a known issue that colonies cannot survive long term on substitutes. Why this is isn't clearly known yet but the assumption is that they are missing some dietary analogue to vitamin C.