You're comparing apples with eggs Fluxinator.
The D&D system is most popular in the US because that's probably the only country where it really broke through as the "pen & paper" game. Baldur's Gate, NWN, Planescape, etc. all use the D&D system and you should not underestimate the sales that will go towards these old D&D fans.
Witcher, on the other hand, wasn't based on a Pen & Paper format. It didn't have a big developer and neither did the publisher advertise it properly. It was a Euro-RPG (like Gothic, Divinity 2, etc.).
And comparing it to Half-Life? Ouch. There's a dozen reasons why you shouldn't:
- Half-Life is a FPS
- FPS appeal to the US market
- Half-Life was immensely hyped before & after release
- the 6 million include all sales from the Generations pack which mainly sold due to Counter Strike
- you were forced to buy the game for it to work on Steam
- they re-released HL a dozen times with different games in the pack
The main problem with games, is the lack of publicity for the more niche ones, that's how it works. Dragon Age looks to be a typical console RPG with the typical flaws, yet it's getting tons of hype. Mass Effect was flawed in so many ways yet it sold loads. The game really is about hype.