Factual error: Anytime a character fires a missile at an airborne target, it's invariably from an M136 antitank launcher. The M136 is designed to hit relatively slow moving ground vehicles and is useless against fast moving airborne targets. (This is because used M136 tubes can't be reloaded and are very cheap to use as props).
Factual error: When Sam breaks her leg, Dr. Keller discusses with McKay the possibility of internal bleeding, then returns to splint her leg and offer her some ibuprofen for pain. A doctor concerned about internal bleeding would never offer a patient ibuprofen, as it inhibits blood clotting.
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