I resigned because you were essentially winning everywhere. You were winning the naval war, the war in Russia & Persia, the air war, the economic war. I tried to make a stand with the Western Allies in southern Persia, but your air superiority made that impossible. The only reason why I was still doing well in Russia was that you were focusing on the south. But I knew once you had taken that threat out, I was toast in Russia. Even without the focus you were slowing pushing me back.
Even so, had I felt I had control of the naval war I might have carried on. But there was very little I could do to your subs, and they were not only strangling my convoys but hitting my infrastructure in North America too. As a result your economy was nearly as big as mine anyway.
As a result I knew the writing was on the wall. Yes you hadn't captured any of the Soviet victory cities, but you were in a position to take all 3 of them by the end of 1943. All I could do at that point was delay the inevitable.
Personally I think you are a very good player, in the top 3, along with Sugar and Korut.