I am sorry for stating this, but It is pretty obvious. I am in contact with Harvey Smith, and I have not asked him this yet, but I feel I know the answer. These guys probably explain it best:
By the way, everything has changed in Arkane, it's been 7 years. Most old team members have left including the most important Harvey Smith. He was the main reason Dishonored 2 was made, because he was really passionate about the game world and he really wanted to make Dishonored 2. He has not left the studio, Arkane Austin was closed so he is no longer a member of it. Only Arkane Lyon exists now, and they are working on the new Blade game, and obviously have no interest in making Dishonored 3, or they probably just don't care anymore.