It's almost 8 years old now and a lot of people were unhappy with the changes Cataclysm brought. Wrath of the Lich King wasn't that hard to gear up and do heroics, lots of AoE fights. Cata made heroics much harder, weakened AoE, and made people use CC more to be able to finish a heroic. A lot of the so-called "Wrath babies" didn't like this and left.
And again, the game is 8 years old. Some people just get tired of it. Maybe it's time for Blizzard to ramp up work on their super secret Titan project, their new MMO they've had in the works.