Obviously, sometimes they do produce rain. In light of that, why is an explanation required? Current explanations have already listed other properties of clouds and that larger clouds (which have more total liquid water) scatter more light and thus the folk knowledge that dark (as seen from the side opposite the sun) clouds are rain-bearing clouds. Likewise, modeling them as systems where temperature and humidity disfavor evaporation leads to the conclusion that turbulent clouds and tall clouds are likely to produce chains of collisions of droplets resulting in larger droplet size and gravity dominating droplet diffusion. But lack of rain from thin clouds seems largely explainable from the context that between clouds and the ground is clear air where the temperature and humidity favor evaporation of any falling droplet. So it's not that droplets don't fall, but that they don't survive in any appreciable numbers to reach the ground.