Because the assassination investigation didn't happen in a vaccumn. If they were investigating whether it was the Soviets or the Cubans, for example, the documents might have contained information about an agent or a broken code that was still useful at the time. Or even just information that could only have come from a few sources. The Western Allies cracked enigma pretty much at the start of WWII and that was a highly classified secret until the SEVENTIES, all on the chance the Soviets would base their own codes on the "unbreakable" Enigma machines.

There's also other things that might have been kept back. Secret Service tactics? Personal information regarding the Kennedys (hell of a thing, if the murder of a close relative results in private issues getting leaked to the public)? Information that could have affected international relations, not directly connected to the assassination?

There's no conspiracy. It's just that one of the most politically charged moments of the century might have touched on other issues that were ongoing. It's probably relevant that the choice to release them sooner was made in 1992. AFTER the fall of the Soviet Union. Unlikely that it would affect relations with a state that no longer even existed.