When your own house is in disorder, you tend to not think too much about stealing from your neighbor. To put it another way, states tend to not get aggressive against their neighbors when they're facing civil war at home. The Jihad messed up the Capellans and Lyrans sufficiently that they were too busy trying to reestablish order in their own realms to spend time trying to pick up loose pieces from the former FWL chessboard. The Magistry had its own issues, as well.
Then, too, the Jihad did a decent number on the industrial planets of the FWL, raising serious questions as to whether the juice to be gained from taking former FWL worlds would be worth the effort of squeezing them. Given how few pro-Capellan rebellions there were in the worlds the FWL took from the CapConnies over the course of the succession wars (unlike in the Capellan March), it would seem that on that flank, at least, the natives didn't really relish the idea of returning to the control of the Liaos, which would have served to make it even more difficult to pick off a few worlds.
No, all in all, the strategic imperative for anyone in a position to grab worlds in the former FWL had too many other problems to deal with at the time, much like no one really snatched at many worlds in the Chaos March; even Sun Tzu Liao didn't try to sweep all of those back up at once, and the post-Jihad Inner Sphere is many times more screwed than at the time the Sarna March devolved into the Chaos March.