I'm going to imagine overwatch is the main reason.
People that were playing it have most likely moved onto other big shooter games now.
Overwatch was huge at one point for quite a long time too.
It's a problem across all of their games at this point. Starcraft 2 is no longer receiving updates and word is that the execs have refused to greenlight a proper sequel multiple times. One of the reasons why some left to form Frost Giant Studio to make RTS games. Apparently there was a spin-off game in the works that got cancelled, too.
Heroes of the Storm had its e-sports league abandoned by the company unceremoniously and the updates have now occurred at a snail's pace. I believe the last time they added a new character was 6 or 7 months ago.
Diablo 3 is already 9 years old and has largely relied on grindy updates of the same old content to keep it relevant. We have no idea when to expect Diablo 4 as updates on that game have been few and far between. Diablo Immortal is a topic I don't even think I need to touch.
Warcraft 3 Reforged was a joke to many people and World of Warcraft has just been failing. Its most recent expansion, Shadowlands, looked like it was going to get the game back on track but it has just gone through the absolute longest period of lacking a content update. The new systems they've added have been a balancing mess and largely boring. The content on offer at launch got stale quickly. Not helping the matter were the stumbles they took into the release of The Burning Crusade Classic. WoW was already on a decline since Cataclysm in 2010 but that was greatly accelerated by poor expansion offerings with Warlords of Draenor, Battle for Azeroth, and now it's looking like Shadowlands could be the next one as well. That would be three of the last four expansions.
Some have accused Hearthstone of turning into a Pay-2-Win game anymore.
Overwatch has basically been set aside for development on its sequel which has apparently been rough. Now its director, Jeff Kaplan, left the company. That alone was another huge sign that something is wrong at the Blizzard offices. That guy was a big face and core leader of the company who clearly loved what he did and his community.
It really is at a point where people should accept that the old Blizzard is dead and what remains is just a shambling corpse. The only reason they continue to have any significant amount of player engagement is due to a lot of attachment to these franchises. I personally have made friends in WoW and we keep playing regardless of how shit we feel it has become because we've already invested so much time into it and it brought us together.
If someone is looking for that old Blizzard magic, the best chance they have with that is looking to those companies the talent moved to. Dreamhaven is one example and probably anyone's best bet of seeing the soul of Blizzard reborn.