n general, there are a few reasons I think nymphs "work" better:
1. >80% of a trout's natural food comes from subsurface items. It's just more of a natural place to find food.
2. Low water: trout are more skittish, not wanting to venture far from cover.
3. Low water, usually means warmer, less oxygenated water: trout move less.
That being said, I still fish dries most of the time in warm weather. Nothing like a spec crushing a big bushy EHC.