Character mistake: Rodney says that one hour in real time would give them months inside the field. This is incorrect because if the ratio of time were 1:250 as stated before, one hour would only convert to 250 hours, which is 10 days and 10 hours.
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.
Character mistake: It is mentioned the previous solar flare was 15000 years ago, and Shepard says that the ancients were in the middle of a war with the wraiths, but as stated in previous episodes the war lasted 200 years and ended 10000 years ago. (00:31:30 - 00:38:10)
Character mistake: In the first few moments of the episode, as the survey team walks through the halls, the German member of the team is wearing his flag patch upside-down.
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).