Why is it that in the 1960s, the Soviet and American militaries were keen on setting up orbital laboratories but the US lost interest before it launched its first, yet the Soviets launched and operated a string of these right into the 1990s?

The answer is that both the superpowers recognized that space was the ultimate high ground and were keen on using it to spy on one another. Unfortunately, there is one problem with taking pictures of the Earth. It's cloudy a lot of the time. This makes orbital reconnaissance inefficient, so the Americans and the Soviets wanted stations with crews on them who could decide when to take pictures, develop them, and then send the right ones back to Earth.

By the late 1960s, the Americans developed an increasingly sophisticated imaging technology, so they abandoned their space lab plans, while the Soviets, who were a bit behind, had to keep using manned space stations for military reconnaissance right up until the collapse of the USSR.