The question is then, why didn't it happen in real life, and what game mechanics are available to simulate those forces?
Anyone with more insight into eastern European dealings of this time cares to chime in?
Wars are a lot more decisive in CK2 than they generally were in real life. The Russians generally didn't have the military capabilities to launch full-scale conquests of each others' domains. Total conquest wars were exceptionally rare, which you can't really simulate in CK2 without making the game insufferably dull.