A lot of reasons.
Inertia – Hearthstone already has a huge market. Being first matters. Also the established base of Warcraft players as you mentioned. All in the past now.
Resources – CPG is a small company and doesn’t have the same development resources.
Marketing – Not a lot is spent on marketing this game. It’s worth noting, however, that HS grew largely by word of mouth. This game did at first, but isn’t much any more. Why is that? There are a number of reasons, but that’s a different thread. All boil down to managment mistakes, IMO, especially things like drop rate and gold nerfs.
Mobile – This is being worked on but my wife, for example, plays HS exclusively on her phone.
Variety – This game has 2 game modes, that’s it. Say what you want about Tavern Brawl, it is something different every week. She is a “filthy casual” and plays TB exclusively. Has no interest in ladder grinding or arena. (I’ve been talking about the need for more game modes for months).
Graphics – Call it a deliberate design choice if you want, to most people it screams “low budget”. One dimension of appeal in a game is being able to make a connection with the characters: it’s hard to do this when an eye is represented by one dot.
Competition – This isn’t the only alternative to HS out there. Some are also very good. There are two new games in Alpha that will provide even more competition to Duelyst. One has a large base to draw from like Blizzard did (though not AS big). The other has astonishingly beautfiul graphics and an amazingly full-fleshed lore and faction design, and it isn’t even in full Alpha yet. And there will be more.
Negative Feedback Loop – This game is on the cusp of entering a dangerous area, where it is perceived as failing and therefore people decide they don’t want to invest time and money in it. We aren’t there yet but we are in the neighborhood, which I am sensing in discussions with others. The mobile release is really important to turning this ship around, IMO. If CPG can get that done, do it well, and get a big PR blitz going at the same time, Duelyst could take off. Otherwise, it is in trouble.