ما هو السبب الرئيس لتحريم الفوائد المصرفية للمودع وللمصرف؟

Not all banking profits are unlawful (haram). The profits from Islamic banks are lawful (halal) because they fluctuate with profit and loss (Mudarabah), and this is a form of trade that Allah has permitted. As for the profits from interest-based (ribaawi) banks, they are unlawful (haram) because they are fixed and do not change. This is a form of usury (riba) that Allah has forbidden, in accordance with His saying, the Almighty: "But Allah has permitted trade and forbidden interest."