Hmm, proof you say? Very well. As far as I know, Fallout 1 sold 144.000 copies within the first two years after its release, and Fallout 2 123.000 copies. Now, considering Fallout being "the mother of all turnbased RPGs", I'd say these numbers are the ones to present to an investor if you want to make Fallout 3 identical.
Let's compare that to Bethesdas latest game. The Big O has, in fact, sold over 3 million copies within its first year, and will most likely pass 5 million before the first two years have past.
Do the math, if you are an investor and you are going to invest several million dollars into a huge game production, do you want a game that sells 5.000.000 copies within two years, or 267.000?
I may be a hardcore RPG fan, but even I'm not willing to throw money down the drain. Oblivion sells, Fallout doesn't. It never did. It's a myth that Fallout sold a whole lot - Baldur's Gate was the selling series that kept things going, not Fallout.
Turn-based RPGs have never sold well, and always been directed at the hardcore fans. 250.000 copies won't do anymore - you could get away with that 10 years ago when games cost so little to develop compared to today, but now it's not nearly enough.
Here are some numbers from 2000:
The entire Fallout series, turn-based, total sales: 267.000 copies sold
Baldur's Gate 1 without any add-ons or sequals, real-time, total sales: 500.000
http://answers.google.com/answers/threadview?id=95704
According to BioWare, the total sales of the Baldur's Gate series is now nearing 5 million copies sold. This includes BG1, BG2 and BG2: Throne of Bhaal.
http://www.bioware.com/bioware_info/about/
Worth knowing about the gaming industry:
Developers do not choose their own games, investors do. Developers do not make any money themselves, they rely on investors to fund them, and unless they can convince the investors that this project is a solid hit when it's released, they get no money. This is a basic description of how the gaming industry works. Of course there's more to it than this, but the essence of it is: Publishers and investors decide which games will be made, and developers are unfortunately forced to do as they are told.