No one knows for sure what happened. But, as far as we can guess - Fantasy Flight lost the license to publish games from ID software. Not sure why, or what happened...but that's the best guess.
So, with that being said the game is no longer being published/produced. Any copies that are out there are the last ones. As for the dice, you could try emailing the customer service and see if they have any dice left. Or look around in hobby/game stores to see if they have any laying around.
One thing to keep in mind though is that there are two versions of the dice. V1 is the original, and all the sides/faces have the information printed on them, and will easily rub off with heavy play.
V2 - the newer sets of dice actually have the information etched into the faces of the dice, so it will not come off. So, if you do manage to find some V1 dice, take some clear nail polish (or something of the like) and seal the faces to give some protection.
A side note, Descent: Journeys in the dark (first edition) is essentially the same thing as Doom, it uses the same engine and fixes some of the problems. Could be worth checking out.