This droplets of water which you find at the surface of the leaf is also known as dew. This are nothing but the molecules of water which we find basically at the surface of leaf during evening( esp .in winter) till morning .We all know that Air can carry molecules of water according to its temperature. So basically if you have a higher temperature then we find more molecules of water in surface of leaf. In evening, we find temperature as very low in winter season especially in evening . And - If condensation takes place above 0 °C its called as dew. So this is the reason !