>>528087069
There isn't enough demand and Rome 2 is in a fairly good state. Rome 3 needs way more time to cook.
Medieval TW 2 is arguably more popular and it makes sense chronologically for it to get a new game before Rome. Med 2 is not in a good state, it's extremely outdated. Particularly with how combat feels and controlling units.
But with 40k on the way I can't imagine them spending too much man power on historical titles, especially with the recent flops.