Because some people don't know any better. The original concept was based on leaf being a harsh smoke. Less leaf, smoother smoke; less Nitrogen, less leaf. But like with every growing technique, it was blown out of proportion by some people. Less N only improves the smoothness of the smoke and decreases weight. It will even decrease potency if the symptoms reach the buds. Exactly how decreased N affects the plants depends mostly on how leafy they naturally are.
Many strains have little leaf to begin with and won't see much results with low N. Other types are very leafy and need alot of N to stay healthy. Since most of these buds are made of leaf material, decreased N can take a serious hit on their yield. There are other factors to consider as well, like vegetative growth in the first half of flowering increasing yield, the fact that the leaves collect light energy to grow the buds, or how many of the leaves will just be trimmed off at harvest anyway.
Whether it's done at the start of flowering or begun halfway through, imo the best way to handle Nitrogen during flowering is to look at it from a minimum point of view. While we pump as much Potassium for bud growth into the plants as they can handle, to the point of burning the lowest leaves, we only give the plants enough N to prevent a deficiency during the 2nd half of flowering. In the first half, they get a bit more. The vast majority of leaves stay green through harvest but towards the end only get just enough N to do so. Being so close to it, the final flushing usually causes N def to show up with some yellowing of the lower fan leaves.
