It was one of the first bans that Smogon did, and happened at the same time as Genesect.
I think it was largely because Mega-Lucario is so unpredictable - nothing can safely switch into it. When you see a Lucario in the team preview, you have to immediately try to work out if it is a Special or Physical attacker.
If it is special, then it will use moves like Vacuum Wave, Aura Sphere, Dragon Pulse, Dark Pulse, Shadow Ball, or Flash Cannon. It also has access to Nasty Plot.
If it is physical, then it might have moves like Close Combat, Bullet Punch, Extreme Speed, Crunch, Earthquake, or Stone Edge. It also has access to Swords Dance.
As can obviously be seen here, even once you work out whether it is Special or Physical, you still won't know exactly what moves it has, since it has access to so many good ones. Most teams would end up losing 1 or 2 Pokemon to it just to work out its move set, as nothing can safely switch into it.
Not only this, but because it can have both Nasty Plot and Swords Dance, then whenever it switches in, you will probably have to switch Pokemon, which allows Lucario to get a free boost to either its Special or Physical attack, depending on which set it is running.
All of this combined with Mega-Lucario's massive Attack stat (base 145) and boosted Speed (base 112) pretty much spelled the doom for Mega Lucario in OU. Not only did it get the stat boosts, but its new ability, Adaptability, means that with STAB Bullet Punch and Vacuum Wave are 80 base power, and Close Combat is 240 base power.
Because of all this Mega-Lucario was thought too powerful by Smogon, and so they banned it.
