Nowakowski revealed (reported via Reddit ) that the two planned expansions were cancelled simply for technical reasons but he did not share additional details on what these technical reasons might be.
For me, these "technical reasons" are that from now on they want to put all their resources into Unreal Engine 5 and focus on The Witcher 4 (https://www.gamesradar.com/the-witcher-4-release-date/).