The rule is that a Eastern European country as defined by the UN should host the COP. However Russia is blocking everybody on the unfriendly country list for Russia. Now they also block the Azeris, who are not even on the list. Not only that they also are a large oil and gas producer. Basically unless they host it in Russia or Belarus, which a lot of countries would block, it ends up with Germany, which Russia also considers an unfriendly country. Not only that you just have to look at EU voting on climate issues to see that the current German government tends to be harder on most climate issues then even the EU members further east.

Basically this is Russia being a dick and getting about the worst deal it possibly could in the matter. The host is a huge part in preparing these events and I can not imaging how Germany would do a worse job then Azerbaijan from Russias perspective.