The Hype started a while back, yes one of the main reasons is the fact its made by Blizzard but its a new genre of FPS, classed as Hero FPS and It is looked at moving into the competitive scene, It has very close competitors in the Hero FPS genre, which before now has only be housed by Team Fortress 2. The competitors are Battleborn and Paladins, both of which are similar, Paladins extremely so and Battleborn play more like a MOBA. Essentially, Overwatch is like TF2 yet TF2 was released in 2007, the genre hasn't had a new game in forever, especially one that wants to jump into the competitive scene.
Hey, Its 'new' who doesn't like 'new'
Plus I think the fact that its Activision Blizzard, its able to market itself pretty well