It is available on Steam in some territories but not all.
This is Activision we are talking about.
My understanding is they will have an agreement with 1 or 2 bricks and mortar retailers NOT to sell the game on Steam owing to the competition it gives them.
Most of the recent Ubisoft releases have not been on Steam in the UK for much the same I believe.
It is in Activisions interests to sell the game on the largest digital platform, the only reason why they wouldnt in my opinion is if one of the major retailers refused to sell their game or give it prominence in the stores if the game was released on Steam.
These are the same retailers who have killed PC retail anyway, ********.
Hopefully, the industry can tell them to shove it in the near future.
They are also partly the reason why digital downloads are not priced as some would expect.
Money grabbing ******** trying to cling onto every last penny they can.