Why didn't they use their aircrafts or aircrafts of occupied countries to deliver supplies to the Soviet Union? Their trains and trucks were having a hard time travelling due to the winter and muddy roads of the Soviet Union. Is it because they needed the Luftwaffe for the invasion that they didn't have enough aircrafts for supplying the front lines? Please answer thanks :))
Transport was a low priority for the German Air Force before the Second World War, and losses in operations prior to Barbarossa (most notably the invasion of Crete) further hampered its capabilities. Air transport was used as far as possible, but was just able to meet priority requirements rather than being a major component of wider supply operations:
"By 1942, air supply operations had become much more than temporary expedients for emergency support of ground units. The critical conditions brought on by the Russian winter, inadequate surface transportation facilities, and tactical exigencies made the airlifting of supplies to large army units - enveloped or otherwise cut off from surface supply routes - a compelling necessity, and, often, the only feasible solution.
The adverse effects of the German failure to organize during peacetime an adequate air transport fleet to serve the entire Wehrmacht, to include air transportation in armed forces logistical planning, and to make corresponding arrangements in aircraft production had become painfully evident." (The German Air Force versus Russia, 1942, Herman Plocher (USAF Historical Studies #154))
The Air Force was able to drop supplies to a small force encircled at Kholm in early 1942, and to supply a larger force of around 100,000 encircled at Demyansk for about six weeks; to do that, though, it had to strip aircraft and aircrew from training schools, and over 200 aircraft were lost in the process. Its success there was a factor in Hitler's decision for the Sixth Army to hold Stalingrad in November 1942 when surrounded, to be resupplied by air. The 300,000 troops ideally required 750 tons of supplies per day, 350 tons at an absolute minimum; the Air Force believed it could deliver those 350 tons by pressing bombers and reconnaissance aircraft into service for transportation. With terrible weather conditions and strong Soviet defences it managed to deliver 8,250 tons of supplies over the campaign, suffering heavy losses in the process, but that was an average of just 115 tons per day, nowhere near enough to sustain an Army. See "The Luftwaffe’s Aerial Resupply of the Sixth Army at Stalingrad" from Lifeline from the Sky: The Doctrinal Implications of Supplying an Enclave from the Air, John Steven Brunhaver, for a fuller treatment.