Continuity mistake: When the Wraith have taken over the Daedalus, Dr. Keller's hair changes from being wavy to being completely straight and back to wavy.
Other mistake: McKay and Jackson enters a room and runs into 2 armored figures, Jackson eventually reveals that they are empty suits, however in every shot shot from behind the suits, the suits are constantly moving, as if they are hanging on strings.
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