There is not "the reason" why many players left WoW but many. One of the reasons is lack of content. After the release of Shadowlands there was only one content patch. It means, you paid 156 €/168 $ for one content patch! And this is not worth the money for many players.
Another reason is, the endgame is basically "raid or die". You have raids, you have very hard 5 man dungeons and nothing more. If you do not want to do those things then the game becomes boring very quickly. There is no housing, no meaningful crafting, no meaningful PvP (OK, FF14 not better here^^), places comparable to goldsaucer are only saisonal events etc. FF14 is way more "broader" then WoW in terms of content.
Another reason is, WoW has too many FOMO mechanics. Very important resources like soul ash or anima have a weekly limit. Yes, Allagan Tomestone of Revelation are also limited per week. But in WoW almost everything what is important has such limits. It means, you cannot take a break from the game or do something different in the game like exploring etc. Because if you miss one week of farming soul ash etc. then you have never ever a chance to catch up because those weekly limits will stop you. This turns the game from "what do i want to do today" to "what do i HAVE TO do today". And after some weeks the players ask themself whether the grind in Torghast to get soul ash for the legendary items was fun or not.
And one reason is also the story telling. WoW had never a good story telling but from WoW vanilla to WotLK it was OK. There was never a MSQ but here and there were many interesting "side quests". And often also nice big quest chains. And the stories and the environment were grounded. The player was a very good warrior and nothing more. But now in Shadowlands you are basically a god slayer. They pushed the story to a cosmic level, you deal with godlike entities and ... when you play the story then you have literally one facepalm every three minutes. You ask yourself how it is possible that this trillion years old godlike entity can act so stupid? And why Tyrande Whisperwind tries to strangle an undead banshee? The problem here is the story does not make any sense. You have an antagonist who plotted many events which started in Warcraft 3! But ingame, he is an Arnold Schwarzenegger-like entity. He only says some meaningless one-liners and nothing more. You have never ever the impression that this guy is a kind of a mastermind. Then the undead banshee who wants "freedom for all" and allies with the main antagonist. The main antagonist uses domination magic, he enslaves souls, later in the story he mindcontrols a human and his castle is named "Sanctum of Domination". Domination everywhere. But the banshee queen did not notice it at all. Sorry but third class fanfiction is more consistent and plausible then this. And then there are so many things unanswered.
Cheers
