Multiple reasons. Dungeons were made to be more difficult, 10-man raids were tuned to parity with 25-man raids, and healing mechanics were changed to require a sharply higher skill level.
Meanwhile, the “WoWdad/mom” demographic – which discovered the game in Vanilla and raided a bit in BC until SWP – had finally become convinced they too could clear content after Strength/Warsong let them deep into ICC.
Cataclysm’s changes hit them like a freight train. They couldn’t compete, clearing the broader pug scene and straining guilds. Ultimately, neither the theme nor the story content was enough for many to stick it out, and by March/April 2011 the face of the game had been changed. 4.2 and especially 4.3 were too late for remedies.
Source: I saw it happen right in front of me. Was great for my own advancement but at the cost of the community.