For decades after WWII, the Germans were using cheap Turkish labor much as American agrobusiness was using Mexicans. When the Turks started demanding the bennies of German citizenship, perhaps the idea of incorporating Turkey within the EU was that they could build factories in Turkey, take the money, and keep them at home.
Then too I suspect EU membership was dangled in front of the Turks to use them as a military buffer against the Soviets. Either way, Germany has swallowed a substantial Trojan horse of German speaking Turks within its borders.