Have you considered the decline of income and job/work hour availability in places, growing recessions and austerity (particularly in Europe), or even getting over playing the exact same game for years, doing pretty much the exact same thing? (What made me quit for 3 months)
Also, there's also friends/guildies that quit, which sometimes kills some small guilds, for the reasons above and the usual IRL changes.
OT: One big part of what made BC and Wrath great was the familiarity. Both of those expansions were sourced from Warcraft 3 and Frozen Throne, and had a lot of familiar elements, themes, places, stories and people. While there isn't another game that can give us new elements (unless Blizz makes a new game in the Warcraft universe that is designed for consoles, hint Blizzard), we need familiar faces in WoW.
To achieve this, what was done with Garrosh needs to be used more often; characters making their way into story, doing things that give us strong feelings about them over time, and putting them in conflict with other characters who have been going through the same process.
This is probably why you get so many asking for Emerald Dream and Aszhara expansions, because those things have been introduced in the game, sound great, and are important.