Heh, it's funny how the article calls it an "embargo" when the west didn't sell coffee to the GDR, but when the Soviet Union didn't do it, it's just "decided".
It's not explicit in the article, but I would guess the USSR didn't have a strong enough coffee industry to be helping anymore.
Edit: as a massive fan of industrial and engineering history, the spotty nature of good Soviet documentation does my head in!
The West wouldn't trade (or let developing countries under its influence do it) and I bet the USSR couldn't procure enough for itself and the DDR.