Both parties just got hold of a lot of new land, which they have never administered, in fact most of it wasn't administered by foreigners since times immemorial.
While large scale military resistance is over, there's still the orderly conduct of life to be rebuilt - elites to be coopted, small rebellions to be crushed, and so on. It may not sound much, but it's far from easy and painless.
In such a situation, peace is the best offer possible. It means for a while, you don't have to patrol the area as strongly; that your neighbor probably isn't trying to get your new subject to rise against you; and it allows your state to focus elsewhere.
Conquest never is free, and most importantly, the cost of digesting new territory is way higher than any media out there would usually suggest (and usually not even for the reasons those media propose, either).