First, there's the microclimate. Just below the Julian Alps, the Collio doesn't suffer extreme humidity for much of the year. The nearby Adriatic Sea salts and modifies Alpine air currents, preventing deep freezing, and reflected light from the sea adds warmth in winter. Second, grapes are primarily cultivated on the hills, a micro ecology favorable to the production of the best wines. Third, the mineral rich earth benefits the wine's taste, and is moistened by the rivers and rainfall.
Irrigation is unnecessary, avoiding overcrowding on the vines and bunching of grapes.
The more grapes, the less quality, I was told. Fourth, wine making is a centuries old family business in the Collio and any wine producer will tell you that experience is the best teacher. Another reason, perhaps, for the pure taste and smooth texture of Collio wines is the emphasis placed by wine growers and local authorities on low use of fertilizers and pesticides. The Consorzio Tutela Vini Collio showed me a computerized information system linked to wineries that transmits information and alerts on climatic conditions and patterns, and bug infestation, to limit use of chemicals only to periods when they are absolutely necessary. Apparently, few wine growing areas of Italy make use of such services.