Alcohol in wine used for cooking adds a harsh edge to the dish. Unless you're heating the booze very fast, when deglazing for example, it takes hours to evaporate away and there will be some left if the cooking temperature is kept low, even for hours. In a slow cooker the alcohol will hardly evaporate at all even if you leave it cooking all day. It's the fruit and acid elements in the wine that you want, not the alcohol. And burning it off is fun!