Surprised with FCO over CIA, o thought the latter would be cheaper?
No doubt they played them out against each other, the airport that offers the lowest fees gets their base.
Wizzair: "Hey Fiumicino, we want to launch a base in Rome but the fees at Ciampino are lower. What are you going to do about it?"
Fiumicino: "We want to offer you fees that are 10% lower than what Ciampino offers if you set up your base here"
Wizzair: "Thank you Fiumicino. Hey Ciampino, did you hear that? Fiumicino is undercutting you by 10%. What are you going to do about it?"
Ciampino: "We want to offer you fees that are 10% lower than the deal you got from Fiumicino if you come to us"
Wizzair: "Thank you Ciampino. Hey Fiumicino, did you hear that? Ciampino is undercutting you by 10%"
Fiumicino: "Okay, we go 10% below them but that's our final offer"
Wizzair: "Thank you Fiumicino. Hey Ciampino, did you hear that?"
Ciampino: "Yes Wizzair, we heard it. Sorry, we can't offer you a better deal. You can set up your base at Fiumicino"
Another factor that might have contributed in their choice is competition. At Ciampino they'd have to compete against Ryanair, which is about the only airline that can undercut Wizzair. At Fiumicino they compete against the higher fare airlines such as Alitalia, that gives them a better relative marketing position. And it means they can distinguish themselves from Ryanair by serving a different airport. Take for example the route to Eindhoven, so far Eindhoven only had service to Rome Ciampino on Ryanair but Fiumicino was unserved from Eindhoven. Now Wizzair will pick up this route, avoiding direct competition with Ryanair.