There comes a point where you move past content. There's no reason to go back to it other than nostalgia, or for the off chance on a slow night that the one really awesome drop that particular boss has might finally drop. It's a fact of the endgame... you work, you put in the wipes, you stock up on food, buff pots, elixirs, flasks, and other consumables, you go in over and over again and struggle and finally get the boss down... and then in a month or two it's trivial and you're sick to death of the sight of him or her. Another month and you're saying "Does anyone really need any Vashj drops anymore?" and a month after that, you're done with the instance.
This is fine, and it's part of the moving on process with content: if you're regularly farming M'uru you probably don't need anything off of Mother Shahraz anymore except for maybe bad luck on shoulders. Some guilds were fortunate (or unfortunate) enough to be completely done with Black Temple and Hyjal well in advance of Sunwell Plateau. Others are just now finishing up with Gruul and Magtheridon and preparing to move into SSC and Tempest Keep, while still others are just now done with those.