The short answer is, because he ate "locusts and wild honey" (Mt 3:4; Mk 1:6).
The slightly longer explanation is that John the Baptist lived a simple life (Lk 7:25) in the wilderness, where he was called from (Lk 3:2) and in which he ministered (Mt 3:1; Mk 1:4; Lk 7:24). Thus he lived off the land by eating these insects for protein, fat, and nutrients (as they were kosher for Jewish diets) and the wild honey for carbohydrates.
Bread and wine are processed foods found in civilization, and John was not like Christ—attending feasts and eating with people in the cities (hence the contrast of Lk 7:33-34, compare v.36; see more explanation in this answer).
So his diet did not include bread for practical purposes in respect to his living outside civilization. No way to make it, few opportunities to buy it (if he even had money to purchase it with), and no convenient to store it, so he trusted God to provide the food from the wild.