If you haven't watched up to Lost season 3 finale, this might spoil some things.
There were four people at the hatch when it exploded. The two outside the button-pushing were Charlie and Eko. Both Charlie and Eko are now dead. In fact, Charlie would have died the the episode just before Eko died, were it not for Desmond's interference. Because Desmond can see the future, and saves Charlie, again and again and again, but the universe keeps course correcting. Was Eko's death also the the universe course correcting for the death that should have happened when the hatch went boom? Perhaps. But more significantly, the two Lostaways inside the hatch were Desmond and Locke. Since the explosion, Desmond sees the future; does Locke?
In the last moments of the Season 3 finale, Jack is about to radio off the island, saving them all, when Locke knifes Naomi in the back and tells Jack not to make the call. But when Jack does anyways, Locke uncharacteristically backs off, saying only "You're not supposed to do this" before walking away. And the way that Lock says that line makes me think that Locke knows, cosmically-speaking, what's supposed to happen, the way Desmond so often knows what's supposed to happen, and this isn't it. Locke's unexpected turn and line delivery is reminiscent of Desmond's lucid-flashback run-in with an old lady at a jewelry store. The old lady seemed perfectly ordinary as Desmond picked out an engagement ring for Penny until he said "I'll take it" to which she replied "no you won't" and proceeded to explain this wasn't the way things were supposed to happen.
No concrete reason is yet available as to why Locke gave up so easily. Although thematically it makes perfect sense. Just like the drama wasn't Charlie dying, it was Charlie choosing to die that made good with the drama and the emotion and the crying. After this long, there's not much joy in forcing Jack and the Lostaways to stay on the island, but there's a lot of meat in their choosing to stay on the island, and in telling the story of why they would make that choice. But that's thematic, a concrete reason remains a speculation.
So consider that Locke has spent a lot of his time, recently, destroying possible exits off the island. Producer Damon Lindelof said "It started back in season one, Locke's desire to keep everybody on the island. Slowly but surely he's methodically picking off every possibility of rescue"1. While Desmond has been delaying Charlie's death until the point where it might lead to rescue, and to his own possible reunification with Penny, his long lost love. Desmond and Locke seem to be the figureheads at the opposite ends of the spectrum. Desmond wants to go and Locke wants to stay. Locke, the man of faith, who once told Jack that he too was a man of faith "just didn't know it yet", (but maybe one-day will?), Locke, who believes in fate and a greater purpose of the island, who believes everyone was brought here for a reason, who was in the hatch when it exploded at the same place and the same time as Desmond (who now sees the future). So what if Locke sees the future too? Recall that the old lady let Desmond buy the ring, despite telling him he wasn't supposed to. Just like Locke let Jack call the boat, despite telling him he wasn't supposed to. "Don't tell me what to do", a phrase so often uttered in Lost, is another way to say free will, baby.
But back to the hatch explosion for one last thought. So I've talked about the four people involved in the hatch explosion, yes? So what about the hatch itself? We've seen a big gaping crater where the hatch used to be, so where did it go? Remember the "meteorite" that struck Hurley's Mr. Cluck's? I sez it was actually the hatch, exploding backwards in time. The beginning of a chain of events that lead Hurley to go to Australia to get on the plane to crash to do whatever the fuckcrap it is they're supposed to do by the end of season 6. I mean, these are the showrunners who put a Van front & center for an entire episode making viewers go "who the fuck cares about a stupid van, give me answers" only to realize 12 episodes later why it was important. How like them to show what happened to the hatch, well before they clue you in that "oh, by the way, the meteorite was the hatch"
1 Quotes were noted in Lost: The Answers recap show