The history of HotS:
It started out being called Blizzard Dota. It had the Dota map, it had items, it had everything that was needed to be concidered a competetor to LoL and Dota 2. It was just a SC2 custom map. Blizzard showed it off on a Blizzcon at an early stage, and people really loved it!
Then they decided to move it out of SC2 to their own game. Here, they started "innovating". They changed name to Heroes of the Storm, removed items, they made some gimmicky maps etc. to a point it could no longer be concidered a competetor to the MOBAs anymore. "Streamline and make it more accessable!". Nobody wanted to switch from LoL or Dota 2 to this game. Basically telling the LoL and Dota 2 players that love blizzard "you are not the target audience anymore, sorry". Sure there were a small amount that switched, but LoL and Dota 2 did NOT lose any noticable playerbase when HotS released. Blizzard went from targeting the MOBA community to caitering to the casual MOBA community that BARELY EXISTS. The game is now basically in "maintenance mode", it's dead.
The same can be said about WoW. "Streamline and make it more accessable!". The people that enjoyed Classic and TBC WoW are today told "you are not the target audience anymore, sorry" and instead they target this casual MMORPG community that BARELY EXISTS. See the resemblence?
And they are wondering why the numbers keep dropping. Wonder if they even see the parallel to HotS. Probably not, so expect WoW in "maintenance mode" in a few years.
TL;DR: Heroes of the Storms is a MOBA made for people that does not like MOBAs. BfA is a MMORPG made for people that does not like MMORPGs.