Matt Grandstaff, Bethesda Community Manager, writes on the Fallout forums that it is nice when you get a demo, but as a gamer, he doesn't always expects one:
"One thing to consider about demos though. For certain kinds of games, I'd say its not as easy to just break off a piece, and say, here's the demo. Sure you can take a game of Madden, let someone choose between two teams, and then make it one quarter. Or as of yesterday, just release a few songs for Guitar Hero III.
For a game like Fallout (or Oblivion), there's a lot of details that have to go into it since the game plays as a sandbox... where do you cut the user off. You might bring up that we have a playable demo that we've shown at events, but from the previews you read, you'll notice that the G.O.A.T exam is never taken, we never decided to save Megaton instead of blowing it up, and so on. Part of the reason for this is that for the purpose of showing the game, they didn't need to flesh out those details.
If we were doing a demo, there'd be a lot of time spent on deciding where a user could go, what quests to include, etc. For Fallout 3, we'd rather commit the time that can be used for delivering a demo into spending more time working on the final product."