The Reason cited by the Obsidian devs was while they would have liked to have the game continue on afterwards, they felt it better to let the game finish at the end of the main quest for QUALITY reasons, as they felt doing a 'hash' job of the 'post endgame' world would be a big mistake.
@ Veraxus5 regarding Spectre's quoted post, I think what he was trying to say is if you finished the main quest having destroyed the Hoover Damn and were able to continue on afterwards and the Hoover Damn was suddenly back in full operation despite the fact you had destroyed it, would be worse.
@Vault Boy. To be pedantic, going by the 'statistics' of the previous Fallout games, Fallout 3 was the only one that had a DLC released that let those who wanted to continue playing after finishing the game, the rest of them (with one proviso) aka Fallout, Fallout 2, Fallout Tactics and the thing that is Fallout Brotherhood of Steel all finished once you fully completed the main quest (The proviso being that in Fallout 2, you were asked if you wanted to continue playing once you had completed the main quest but even there if I remember correctly, you have a hard coded time limit of about 13 ingame years). Personally I think that the Broken Steel DLC for Fallout 3, broke if not shattered Fallout 3's game balance as in made it far too easy for all of your characters to become virtually identical clones of each other as far as their skillsets went being one of the main major problems (Hence why I tend to refer to Broken Steel as Borked Steel).
Back to the main purview of the OP, the hardcoe Achievement is given as has been mentioned earlier in the thread is for Completing the game (Finishing the Main Quest) with only playing on hardcoe, as if I remember correctly while you can choose to turn the hardcoe mode off at any point during a playthrough, you can only turn it on at the start of a playthrough. Although even if this isn't the actual case, I think the achievement would be disabled for a playthrough if you turned the hardcoe mode off at any point during that playthrough.
