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.
Answer: Because even all 4 Naquadah generators (one was destroyed) together is only enough to dial Earth for approximately 1.2 seconds (As seen in the episode "Letter from Pegasus"). Further, if Earth had found a ZPM, they would dial Atlantis and let them know. After all, the SGC has no other use for a ZPM than to dial Atlantis.
Grumpy Scot