Wine vinegar was chosen because it would have been readily available. Olive oil was used for similar reasons, and because it would complement the flavor of the olives.
Olives and Capers in one dish, with vineger and oyle.
Thomas Dawson, The Second Part of the Good Hus-wives Jewell, 1597; Arwen Southernwood, The Stewpot Period Culinary Guild (SCA)