Britain and the U.S. had made up their mind to back Tito despite successes the Chetniks were having. The BBC was deliberately giving credit for Chetniks attacks against the Germans to Tito.
The Chetniks were purposely deprived of aid from the very beginning despite the fact that they were the ones who initiated the uprising against the Germans.
As for Belgrade - the Germans were hanging Serb civilians daily in the city center at the beginning of the war and doing this also in other Serbian cities of Pancevo and Novi Sad.
The local German Volksdeutsch were the ones who joined and cooperated with the Nazis.
Draza also took arms from the Germans and Draza’s Chetniks took part on the Axis side in offensives. Collaboration.