The reason they said was that allied bombs when being developed were typically dropped onto harder ground than that which covers most of Germany, and many bombs just literally sank into the loam.
Berlin is in an area of low-lying marshy woodlands with a mainly flat topography, part of the vast Northern European Plain which stretches all the way from northern France to western Russia. The load bearing ability is such that Hitler had a 12,000 ton test building erected to see if the soil would be able to support the planned buildings of Germania. The test piece still exists.
