By all accounts, 6th army was the cream of the German army during WW2, having the best equipped and trained soldiers. Why weren't they put into action on the Eastern front until after Barbarossa had failed?
6th army was the cream of the German army during WW2, having the best equipped and trained soldiers
This isn't really true. 6th Army was, for it's time in Case Blue, the largest Germany Army ever assembled, the headquarters having c. 330 000 troops under it's command, but that was about it. It was even beaten for that title in 1943, with the Kursk Offensive, by Model's 9th Army (c. 335 000 men).
Why weren't they put into action on the Eastern front until after Barbarossa had failed?
6th Army fought in Barbarossa, under the command of Walter von Reichenau (Friedrich Paulus was his chief of staff). It's record, like that of Army Group South in general, was mixed due to heavy Red Army resistance.