In Hulk (2003) why did one person survive but another didn't?

According to the Official Novelisation it was a mixture of factors.

• David Banner absorbed the Hulk's power. He was able to cope, but only just and at the cost of merging with the Hulk entity.

• Bruce Banner (as junior partner in the merger) was simply thrown out of the gestalt entity and fell into the water, where he was protected from the explosion. Later he emerged and went into hiding.

• When the nuke went off, it gave David Banner too much power and he overloaded and died.

• Bruce's power drain proved to be only temporary. After a period, his Hulk form reasserted itself (as we seen in the final scene in the Amazon).

The Hulk seemed to dissolve, but Bruce Banner could be glimpsed briefly inside the falling shape as it dropped into the lake. His father, victorious, towered above the mountains. He saw on the horizon a fleet of puny Stealth fighters and jets making their way toward him, and he laughed and laughed, and his laughter resounded like thunder.

Then he paused, and looked down at his stomach. Swirling energy radiated into his whole body making it bigger, bigger. He thrashed about, looking for his son or the Hulk, and began to scowl.

“You!” he shouted to no one. “The reaction—you tricked me! Take it back! It’s not stopping!”

Nor was it. It spun out of control, the different energies colliding, his body absorbing everything, the moonlight, the air, the wind, and when there was nothing else, his body—seeking new energy sources—found the largest one around: itself. His body literally began to devour itself, the effect flowing from the middle and surging outward, and as the father clutched at himself and screamed and howled, a voice sounded in his head, and it might have been his own, but it sounded like his son’s. And the words—the parting words from his offspring—burned into his fevered consciousness.

. . . things fall apart . . . the center cannot hold . . .

David Banner stumbled to the top of the mountain, and this time he didn’t notice the fighters swiftly approaching from behind.

And in the far, far distance, Thunderbolt Ross looked at his daughter as he gave the final order. “Gentlemen, release.”

The thermonuclear missile took off from one of the planes, heading straight for the father who continued to grow and distend in an agony of energy. Something warned him at the last moment, and he turned and saw it coming. For a half-second a grin split his face as he anticipated more energy to absorb, but then he realized, Too much! Too much!

. . . the center cannot hold. Best wishes from this rough beast . . .

The missile struck him and his center shredded and blew apart, unable to contain it, as a massive explosion—an explosion evocative of that which had haunted Bruce and Betty’s dreams for as long as they could remember—engulfed the sky.