Why did the allies decide to concentrate first on the war in Europe?

Churchill and Roosevelt knew that Hitler's armies in Europe were the most dangerous and causing the most harm. Since there were limited resources and manpower, the Allies didn't want to divert and more effort than absolutely necessary to the Pacific theater. The Allies also wanted to weaken German forces in Europe so that they could invade (in what would become Dday), which was an absolutely crucial part of winning the war.