Because it's proof that a new game is coming out, something a lot of us have been longing for.
Same thing if Blizzard release a trailer like this with Warcraft 4. A lot of people would become really happy.
For me personally, there is only two companies that would get me remotely hyped with a title alone. And that is Blizzard. And Possibly CDPR.
Because unlike Microsoft, has not released any bad games.
I do not play Starcraft 2, but I enjoyed the campaign.
I do not play HOTS anymore, but I got 400 games played.
I do not play WoW right now but I got a few hundred days of playtime.
I do not play Diablo 3 anymore but I got a thousand hours or so played.
I do not play Overwatch but I got to level 136
I do play Hearthstone casually now but used to play it a lot before, 9000 wins or so.
I do not have any way to track time played for the classic games but I think you get where I am coming from.
I trust in a company, not in a title.