Probably due to lack of long familiarity with those alloys, although the couple of mentions I saw of the issue weren't specific. German chemistry (explosives, etc.) was superior to the Soviets', but the Soviets made better use of specialty metals (suspension and recoil springs, gun barrels, etc.), which the US only matched years later when computer technology and improved spectrum analysis techniques allowed for rapid metallurgy advances in a relatively short time. Previously, it was mostly trial and error to get the right combinations of specific metals, and difficult to reproduce when anything changed in terms of metal purity, etc. The Soviets had plenty of experience at it, Germany had less.
The machining requirements on the T-34 were far less exacting than on the Panzers, but that's another matter completely. When the massive tight-tolerance lathes required to produce turret rings were about to be overrun by the Germans, and mostly destroyed to prevent capture because they couldn't be moved, the Soviets switched to novel methods of electrical arc cutting machines using wooden fixtures in some cases. The tolerances were way off, but the metal was sound.
It's been so many years since I ran into some of this material that I can't pin down a source anymore, sorry. Some wasn't even specifically about WWII, but about electrical cutting and welding, aircraft development, and other "loosely related" material, which happened to mention the issues in our context.