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.

>Dental caries is the quintessential disease of civilization, a disease which became prevalent with the start of cereal agriculture and rampant with the start of industrial sugar production [1]. A body of evidence supports the hypothesis that a diet leading to dental caries also leads to chronic non-communicable diseases [2].
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>Most authoritative organizations aimed to protect public health ignore this evidence and take the view that dental caries is the only adverse side-effect of their high-carbohydrate nutritional guidelines, a side-effect which can be addressed with universal fluoride recommendations