Because if you don't crush a rebel movement if festers and can lead to other anti-government groups to get "the wrong idea" and cause more sucession.
And as for bitter fought war, I look at Filtvets military they wouldn't stand much against a couple AFFS RCTs with Warship support.
There are very few problems in the battletech universe that CANNOT be solved by the application of warship support.
Simply look at the TH and the SL.
Killing it with a battleship was like their main method of diplomacy. Or the threat thereof.
The unthinking application of force sure ensured the Hegemony and the Star League's long term survival, right? ^-^
The takeaway point here seems to be that, despite folk wisdom to the contrary, sometimes it's just not worth landing in a regiment or tossing around ortillery. What would crushing the FC and dragging them kicking and screaming back into the Suns actually do? The remaining seven worlds are going to be less useful after an intense war, they're not that useful to begin with, and it's actually an outlet for rather than a cause of secessionism, in some ways.
Sure, the inner sphere is often ruled by megalomaniacs, but the Davions have, historically, been less interested in hegemony just for the sake of it - especially when they can't construct an ideological crusade around it. (And I say that despite their tomfoolery in Concordat space centuries ago - unlike, say, the Cappies, they seem less likely to foster a generational sense of revanchism).
At the very least, conquest by war is less desireable than economic, social, or political options. Consider that the FedCom was achieved through marriage, not through hitting the other guys with warships. Or that initially the Davions didn't fall on either Hegemony OR Republic territory until the other spheroids began it.
