Might have to do with the fact that East Germany's Uranium deposits dried up sometime between the late 60s and early 70s (but of course there was also the global oil crisis around that time). In the early days the GDR provided most of the Uranium for the Soviet nukes, and there was constant haggling about who pays what (usually the GDR) and how much profit the GDR was allowed to take (usually not much). Cheap oil and gas might have been a part of the deal, until West Germany also discovered that the Soviets were willing to sell cheap to the "class enemy" during the 1970s (e.g. that problem of (the whole of) Germany becoming dependent on Russian oil and gas started much earlier than most people realize today.

Also related fun fact: the GDR started to refurbish and use old steam locomotives again as a result of the oil crisis. I remembered seeing those regularly as a child, and then later wondered what caused this anachronism.