Star Trek VI is more allegory than science. Everyone knows that it was meant to symbolize the Chernobyl incident. It was more dramatic to have the Excelsior "flipped" than it would be to have the equivalent of Geiger counters in California picking up residual radiation being carried through air currents. Also, like Chernobyl, it did not doom Russia, but it was a symbol of the waning of the soviet communist system. Likewise, the Khitomer accords are equivalent to Glasnost. Since we didn't know at the time Russia would open up, only to turn into the kleptocracy it is today under Putin, we had no way to extrapolate what happened to Klingon culture in the gap between Trek VI and TNG, and so that period was never filled in.