As the Winter Knight, with all the implications of his actions reflecting on Mab (most notably in the confrontation with Marcone), he might be restricted in what exactly he can do about the Fomor. I'm not sure what their political status is - "Even Hand" suggests they are signatories to the Accords, while "Bomb Shells" would suggest they might not be, or might not be for long. Nicodemus was a signatory in his last appearance, but specifically says that he no longer is by this book, so the Fomor might have forfeited that status with their treachery against the svartelves, as I assume Nicodemus did with his violation of a truce agreed to on the basis of the Accords. In any event, it's easy to imagine political complications that would prevent him from just hunting down the Fomor and kicking their asses until they bail on Chicago. At the very least, White Council stuff, or his Warden (of DR) duties, and the rising threat of the nemesis contagion might prevent him from fighting a whole new war, as he did with the Red Court (which lasted 9 books). Or maybe another power starts kicking up a fuss, preventing him from going to town on the Fomor until one or the other is dealt with diplomatically.

In keeping with the themes of the series, there will almost certainly be complications to the exercise of any significant amount of power, preventing Harry from simply embarking on a crusade to reestablish Chicago as a safe spot under his protection.