1.) They are on top of the world. They don't need a regular demo to hype the product. That said, why bother with a tech demo just so non customers can waste their time with support issues? Who wants to support non customers?
Its not like they have an entirely new engine. I'd say much of the code is from Oblivion yet and certainly fallout3, just with improvements and enhancements. They have at least 100 guys in house developing the game and countless testers as well on a very wide variety of hardware. The chances of a hardware bug coming up that is so severe that programmers cannot fix and actual paying customers return the game are slim to none.
2.) The time to make a tech demo is substantial. Why not spend those hours improving the game? Its wasting money for something they are confident will not be an issue anyway.
3.) Also, the majority of their customers will be on the consoles. And the port to PC is quite easy from a console, and fallout3 didn't have too many problems, if it did I never noticed the. So really this is just another whiner thread that is wasting bandwidth.
4.) It's also such an epic game that the chances are if someone does buy the game and it doesn't work, most people would wait for a patch than take it back.
Making any sort of demo would be a complete waste of time and money. They should get on with making DLC's and deal with any bugs after the game ships, they know what they are doing and it will be minimal I am sure.