It seems like a very constant complaint plenty of people have about Victoria 3, that POPs are only on a state level and although players can split states, the devs have confirmed that it will be relatively rare.
With a lack of confirmation and any direct information regarding this, the community may only speculate on this matter. However, using the information gained from some dev diaries, one can extrapolate quite a lot.
It's safe to say the main reason for this, is because of performance. This is the only Paradox series with POPs as the core focus of the game, with such depth being given to them, compared to Stellaris, and it will have 4x more POPs than Victoria 2, so performance is obviously a concern for many people. Remember, the game will have more than 13k provinces and a billion POPs at game start, which will roughly double by the game's end.
Victoria 2 had a fourth of the amount of POPs that Victoria 3 will have, as well as much fewer provinces. Even then, Victoria 2 can suffer from performance issues when you use mods like GFM and CWE. The latter even has an option for diminishing the amount of POPs so you can increase the performance.
Another thing to mention is the fact that Buildings are built at the state level, so which buildings does a province get when it's split off? Or would they be built in provinces now, with the player choosing? If that choice is going to be meaningful then, a lot more would need to be simulated so that when you build an iron mine in a state, it would make more sense to build it in a specific province rather than another, and that would change for a textile mill.
Without that, the player would just build all the buildings in the "safest" province or something like that so that if the AI wins a war and takes some provinces on their borders, they don't take away any buildings.
With Victoria games having such an original mechanic and gameplay, not having provincial information will severely inhibit the immersion. While all of these issues are substantial, perhaps a middle-ground can be found by making provinces give some basic information compared to the state level, reducing the load on the calculations made every "tick".