HotS is (relatively) dead because:
1) Way too late to the party
2) Tried to out-League League with the removal of complexity, doesn't work, there has to be a set amount of complexity for players to feel like its worth investing in, League players had the same view of HOTS as Dota players had of League, that says a lot
3) Same business model as League, which is bad, because a player only has so many hours in a day and they can't grind League unlocks if they also have to grind HOTS unlocks, meaning less time "shopping around" as it were, meaning less chance of being retained