This is...reasonableNot established, but as you say, reasonable. Banner & Marks never performed a thorough assessment of the damage to the chamber, so for all anyone knew,, the Hulk might have created a "time bomb" of sorts with any of the damaged power lines running from the chamber to other systems in the lab.
Remember, at the time of the pilot's events, Banner retained no memory of his actions as the Hulk, so he does not know if he had any direct responsibility in the death of Elaina Marks. In fact, in episodes to follow, David says he cannot prove he was not responsible for her death. That places him in a dangerous position; if he "returns from the dead," with no evidence conclusively proving the cause of the explosion, he likely fears he could face charges (whether or not that happened did not matter--he felt that was a serious problem).
Moreover, there's the McGee problem: as you say, he's successfully convinced the world that the Hulk is responsible for the deaths, but if Banner returned--a man McGee suspected was responsible for the Hulk's existence, he could add fuel to the fire with his other eyewitness account, as he explained to Banner & Marks' colleague Ben:
McGee: "Look..doctor. I saw what that creature did to a steel chamber. And I heard your friends talking--they were scared to death it was coming back."
So, McGee was not just suspicious of Banner's involvement, but he overheard the two key people discussing the unwelcome possibility of the Hulk returning. The same Hulk that ended up "murdering" at least one person. No one has to believe McGee (considering his "unreliable source by association" of being a National Register reporter), and with Banner himself being a living witness, Jack's account may not go anywhere. Who knows, but we do know Banner was concerned about his actions as the Hulk that night, and for that reason, he feels he cannot "return from the dead."

Not to mention that if David had revealed himself to be alive, he would've been questioned about the circumstances of the explosion and wouldn't have been able to give an answer without revealing the whole story. And then, not only would he potentially have been deemed culpable for Elaina's death as TREK_GOD_1 says, but he probably would've been whisked off to the kind of black-ops government lab that's the bugaboo of any fictional character with unusual abilities.
This could become an interesting development. Remember when in The Powers of Matthew Star the main characters became government agents?
Errr... strike that. Never mind.
