If you go with with "the Klingon Empire has fifty years of life left" line from Star Trek VI one might surmise that the Narendra and Khitomer attacks happened at a particularly weak time for the Klingons.
The Romulans already had some type of Warbirds yet I doubt Vorchas were in service yet.
Maybe a calculation was made that a full military response would have resulted in greater defeats and or dishonor.
I also like the theory that house Duras was powerful and prevented hostility between the two powers.