Because the Germans would hold a shipping lane on their side of the channel open for warships those longer supply lines more vulnerable to interdiction by uboats and surface ships. ITOL the Germans only stationed small coastal uboats in Belgium, and these operated not far beyond the Dover mine barrage, but ITTL larger patrol uboats could be based on the far side of the Dover narrows and operate deep into the channel. The RN/NM would have to expend resources to protect these longer supply lines, which again would come from the resources that IOTL were deployed in the trenches.