The fall from grace started with the advent of paid services, and in game cosmetic shop in WoW and all of the issues surrounding the launch of Diablo 3 (let alone the disaster that is OWL/Overwatch, complete abandonment in HotS, etc.)
What finally killed it for me is the role-based queueing. It locked in the developer-proscribed 2/2/2 meta and made the game feel very static and uninteresting. Previously, if you saw a gap on your team you could switch to a completely different role to fill the void. Though even before role-based queuing there was a (very annoying) portion of the community that believed you had to be playing the high-end "meta" even when you're at the lowest ranks or playing casually and would complain endlessly if you did anything different.
Really just highlights Blizzard is terrible at esports. Lots of examples here for MDI, AWC prize pools vs. investment required to be successful.
There were huge org buy-in fees (in double-digit millions) for even fielding a team, completely unheard of for an esports league, and it was coming out of the gate with almost zero community competitive scene to back it up. Esports scenes thrive on an amateur/semi-pro/pro feeder system and for the parent company to dump a hundred million dollars at the top end without waiting for organic growth at the bottom smacked to me of a cash-grab by Blizzard. Also, Blizzard established itself as a monopoly with exclusive rights to create tournaments, which really rankled with the semi-pro players.
Aside from the tedious balance issues that resulted in us watching an entire season of OWL where top-tier hitscan dps players were forced to play tank, the whole top-to-bottom emphasis on "support team [geographical location] [adjective]" did nothing to foster support for particular squads. Esports leagues are rootless - there are no Wolverhampton Wanderers or Gunners on the Internet - instead people follow specific players for their individual playstyles and skills.
The most hilarious example of the most tone-deaf enforcement of this was when Seagull (one of the OG beta testers that somehow exploded and got thousands of viewers on Twitch) got signed to Dallas [whatever adjective] and proceeded to spend the next 6 months benched (and offline on Twitch) because his skillset wasn't meta. He then spent a single season in competition, quit, returned to Twitch and made orders of magnitude more money from doing whatever he wanted than he would have got in the League.
The thing that sealed the League as a scam for me was the introduction of linkage between a Twitch account and your OW account - you could gain in-game tokens to buy cosmetics from time spent watching OWL on Twitch, but this was trivially defeated by setting the stream to 160p resolution, muting the tab and continuing to do whatever you wanted. At that point it was obvious it was all about gaming metrics and not making a game, and I got bored and left.
however it's managed really, really badly
it's a billion dollar franchise and can't even put out a new map for the regular game mode in 24 months
(or slightly alter two broken ones in 12 months, hlc + paris)