Most of the planes which landed on Iwo were performing training missions.
Also, the re-raising of the flag was not the moment of conquest, as shown by the death in later fighting of several of the Marines and Navy Corpsman who re-raised the flag.
The Army fought on long after that flag raising. The Marines were crucial in the initial landing, and then, what was left of them were switched over to Mt. Suribachi.
The soldiers, corpsmen and Marines were fighting without the knowledge of the Atomic Bomb. For all they knew, we would have to continue on to take over the entire Japanese Islands. Iwo Jima was their task, for that moment, and they, if they thought about it, would look forward to many other such tasks.
The Japanese also didn't expect the war to end soon. They intended to continue to fight, and were willing to spend 30 million casualties if they could inflict 1,000,000 US casualties. Their goal was a negotiated treaty. The US was reading the Imperial Japanese diplomatic codes pretty regularly, reporting the information gained as "MAGIC".
Additionally, the US arsenal of nuclear weapons after Nagasaki wasn't nearly as continuously supportable as was the island hopping campaign in bringing US firepower to Japan's doorstep.