I would like to know why and how Halo Wars 2 hasn't come to Steam yet. We got Halo Wars 1 and that was a re-release handled by 343 since Ensemble is defunct. So given 343 worked on Halo Wars 2, I feel it could make sense to release it on Steam too, though this may be up to Microsoft if they wish to do just that. If they do release a definitive edition of HW2 by giving you all the DLC, a few new multiplayer maps to play on, and the ability to choose any campaign mission from the get go. To repeat the question at hand: will Halo Wars 2 ever get a Steam release?
All comes down to time and resources, at the moment 343 don't have either to devote to HW2 - majority of the team are working on Infinite with small teams working on MCC and Halo 5 sustain. There's always a chance it might come to Steam in the future, but it won't be anytime soon