Primarily, it was found to be able to run too many viable sets, including the Dual Screen (Reflect/Light Screen/Stealth Rock/Taunt) set, which was found to be too centralizing to the metagame.
The idea that it was centralizing (which is disputable) was a bit of a side effect; nowadays we would say it's Uber under the Support Clause, since it not only could get up get up Stealth Rock and deny the opponent Rocks, the Dual Screen set made the Gliscor/Metagross Baton Pass strategy almost unstoppable. In fact, it's a classic example of what the Support Clause actually means.