Because they can only work on so many games at once, keep supporting and expanding their games very long after launch (which is awesome but drains a lot of resources that could have been used for making new games) and they decided to launch a new version of Rome before Vicky. Between EU IV, HoI IV, CK II, Stellaris and Rome, I suspect that the Paradox studio is already stretched to the limit. A new Victoria game is probably planned already, and likely they already have plenty of ideas and plans for it, but they just don't have the resources and time to make it yet. Which means that it will be coming, but it is still quite far in the future. At least, that is what I think is happening.