There are several reasons not to have a demo for your game in this demo-hungry period of gaming history. There are time constraints that come into play, not wanting people to know your game sucks before it comes out, and in the case of Fallout 3, just too much world to hack off a piece.
"When you build it as one thing, there's no way to portion off a section and have it stand on its own without putting the whole game in the demo, which we're just not going to do," said Hines. "And it doesn't really capture the fun of a game like an Elder Scrolls or a Fallout, where you can go where you want and do what you want. So no demo, sorry."
It's like eating a strawberry off the top of a cake. Sure, it tastes great, but there is no way you'd know how the whole cake tastes just from eating a strawberry...and what if you're allergic to strawberries? Instead of picking it off and moving on, you'd assume the whole cake has strawberries? And what if you took an analogy too far? That's when you toss out three new screens and hightail it out of there.
Fallout 3 demo not possible [Eurogamer]