Found something up there they tried to keep secret? There’s been rumors and theories that the moon isn’t a natural satellite. With theories ranging from a gargantuan sized ship to it being hollow and filled with caverns containing the records and ruins of other Civilizations both human and otherwise predating our own. Research Bases would prove a tempting target if you seek to prevent the foe from gaining an advantage.

Also natural strategic advantage. As Nukes launched from a Space Base would bypass ground based tracking. And interplanetary tracking is spotty at best. Especially in the late 70s/80s. So the Soviets may not have had any idea they were about to be hit until any launches penetrated the atmosphere.