As far as I am aware, its because thats where the talent wanted to be. Johan and Fred and a bunch of other folks have moved there from Stockholm. Also as a growing global company under Ebba’s leadership I think Paradox is enjoying the process of starting new studios wherever they see a real opportunity to create games that can resonate with the new broader audience.
Its similar to Berkeley/Bay Area with us. This is just where we are already and where we like to be, we have access to (arguably) one of the best talent pools in the world so Paradox was delighted to start a studio here, less delighted by the local cost of living, but thats true for all of us :) Likewise Barcelona is within the EU in a great location so talent will hopefully be attracted to that.
Hope that helps!