there are more wild yeasts inside the apple, than on the skin, both good and bad! proper use of sulpher, based on acidity of the juice, will kill the bad yeasts, and after a period of time, the good yeasts will multiply and take over the fermentation. There are also wild yeasts on your press, rack clothes, ciderhouse walls, etc., after a year or two of production. When your ciderhouse and equipment are new, it can be hard to ferment with wild yeasts, as there is not a large quantity on/in the apples.
I recommend reading Andrew Lea's site @ The Wittenham Hill Cider Pages for great info on making craft cider.