Why did Niels Bohr initially think an atomic bomb could not be built during World War II?

Following the discovery of nuclear fission, Niels Bohr was fully aware of the theoretical possibility of creating an atomic bomb. However, he initially believed that the practical challenges involved in developing such a weapon were so prohibitive that it could not be realized until well after the war had concluded.1

Bohr maintained this conviction even after being informed of German efforts in 1941. It was not until he arrived in London in 1943 and was briefed on the actual progress of the Allied atomic bomb project that he changed his mind regarding the project's feasibility.1