Wasn't closed per se from what I remember reading. Think it became a normal questing zone with STV around the release of Cataclysm rather than remain a raid instance, probably only a temporary measure so the place got used more whilst they spent time updating it all. Would be my guess.
Not that I can ever see myself going back to WoW but it's good that they're doing stuff like this to the older instances. Whilst they'll always release more instances with each expansion so the game's a never ending quest for gear it's nice to see the older instances get some love and attention and be raised up in level rather than be left behind and forgotten about. One of my worries when I still played was thinking that instances like ZG, MC etc would become obsolete once the level cap was raised and higher level places/gear were implemented. Good to see that's not necessarily the case.
"My name is Ozymandias, King of Kings:
Look on my works, ye mighty, and despair!"
Nothing beside remains. Round the decay
Of that colossal wreck, boundless and bare,
The lone and level sands stretch far away.