I doing think it was an one thing, but more-so a build-up over time - a death of a thousand cuts, if you will:

It was a cultural moment generally, just think back to all of those celebrity commercials (“I’m Mr. T and I’m a Night Elf Mohawk”). All cultural moments pass eventually.

The third expansion (Cataclysm) was quite weak to begin with; coupled with a lack of content in the tail-end of the second (Wrath of the Lich King), which itself was incredible - narratively wrapped up the story that began all the way back in Warcraft 3.

So a lot of people chose that time to bow out of the game, as it required a fair bit of time dedication and seemed like an appropriate time to do so - given the narrative pay-off.

Lastly, the introduction of a number of game tools to automate the group composition process meant that the impact of player reputation on servers was severely diminished. Before then, there players who were toxic (stealing items, intentionally killing the group, failing quests) were infamous on a server.

Once this tool was further opened up to allow for groups to form across multiple servers - the sense of community was shattered as you would have no way to know if the person from another server was good/bad etc. it stopped being about bringing in the individual player, and just getting a body in to fill a role.