Character mistake: General Landry says that the Asurans would need to rewrite McKay's macro to gate to any planet in the Milky Way. In fact they wouldn't have to bother with the macro, because they could simply gate directly to the Milky Way from a ZPM-powered Atlantis.
Continuity mistake: Lt. Colonel John Sheppard is packing his belongings up and his skateboard is against the wall at first, then it is on top of boxes.
Chosen answer: Because the ZPM allowing them to dial Atlantis was being used at the Antarctic defense outpost. They couldn't risk making Earth vulnerable to attack from Anubis by depleting it with repeated dialing to the Pegasus galaxy. Once they knew Atlantis was in serious danger, they had no choice.
Grumpy Scot
ZPM from Antarctic was depleted in first dial, dial in Siege was done via ZPM from Egypt (SG-1: Moebius Part-1/2).