I think Turkey wanted to take the land for keeps - same as Afrin and Idlib.
It those other areas they conducted massive ethnic/sectarian cleansing, to push out Kurds and Shi’ites, and pumped in a lot of Sunnis - especially the militant Wahabbi style.
They set up essentially Turkish Government in those locations - Turkish Post Offices, the whole nine yards. I believe that when the final lines settle out and the shooting stops, they plan to hold referendums on joining Turkey, and annex them permanently.
So the Syrian Government (Assad) and the Syrian Kurds are naturally on the same side against Turkish aggression - they just could not agree among themselves who the boss would be, and how things would work out. The Kurds were using their US backing to drive a harder bargain for autonomy from the Ba’athist Government.
They both knew that they would have to deal with Turkey and Erdogan after ISIS was cleared. That day, this new phase of the war, has come.
Idlib is not secure. I don’t see how it could be without Russia’s agreement, which I think they won’t give out of concern for Assad.
Repopulation of N Syria- which I think Assad would agree to for the refugees( and neither Assad nor Erd care about displacing Kurds)- would be nearly as good for Turkey as a conquest.
And much, much cheaper.