Flour products and sticky carb-rich foods that stick to the teeth result in increased biofilm production, and the resultant shift in pH from bacterial acids causing degradation of the enamel. Fluoride works bc it is incorporated into the enamel, replacing hydroxyl groups in hydroxyapatite to form fluorapatite which is more resistant to bacterial acids.

edit: among sticky foods, this also includes nuts/seeds (and nut butters by extension, incl. legumes such as peanut), as evidenced by archeological evidence suggesting that hunter-gatherers that relied upon such foods had a high risk of caries development. In contrast, hunter-gatherer peoples that ate primarily animal produce had a low rate of caries development until the introduction of refined carbs.