No simple answer. It comes down to four things:
1. The game itself.
Forget quality - we know shitty games can sell as well as good ones. But some games will simply have a limited appeal. Zack and Wiki is a perfect point. "A new point and click adventure!" is not going to get the average gamers blood going. The good thing about Z&W is that this weakness was evident from day one, it was budgeted accordingly, and should not need many sales to make it a success. The problem is when normal, or even excessive, resources are dedicated to such titles (Viva Pinata?)
2. Timing.
Don't release it up against heavy weights like Mario or HALO. Again, Zack and Wiki is a prime example, but I can also point to Beyond Good and Evil.
3. Marketing.
Part of this is game design and ackaging - really, does anyone look at the characters and box art for Zack and Wiki and think "Boy! What loveable, zany characters! I must purchase this game!" Compare that to the creepy diving suit of Bioshock. While it doesn't ensure a purchase, I suspect it got a lot of people to at least pick up the box and see what it's about.
Then there's just getting the word out about your game, and convincing people to buy it. This can be especially tough in a crowded field - how many average gamers really know the differences between the various WW2 shooters out there? I still can't find anyone to tell me why Killzone 2 is going to be any different than dozens of similar titles that have been released. Gamerace's example ior Prince of Persia is a perfect example of how to do it right. Despite the fact that it's an adventure game set in booby trapped temples, PoPs ancient persia setting and, most importantly, Sands of Time feature immediately distinguish it from Tomb Raider.
This is why established IPs really are so much more valuable than new ones - they're already presold, and consumers have past games as a measuring stick. Really, as awesome as SMG is, just how many people would have grabbed it if it "Puppy Pete's Planetary Adventure"? Does anyone think it a coincidence that Knights of the Old Republic was a platinum seller while Jade Empire (by the same developers) went straight to the bargain bin?
I really think this is a problem with Uncharted. If it was Lara Croft, it's presold. But just looking at the footage and screenshots in ads doesn't convey what makes the game good enough for a purchase. Slap some GOTY labels on the box, and maybe it'll move faster.
4. Dumb luck.
Sometimes a games fails despite doing everything right. Eternal Darkness seems to me the quintessential example. It filled a much needed niche on the GCN, the horror genre was near it's peak, had great reviews, was published by freaking Nintendo (really, when has anything else ever been needed on a Nintendo console?) and the sanity meter and historic scope gave it marketable hooks to distinguished it from Resident Evil, Silent Hill, etc. Thankfully it's miserable sales have not yet killed the prospect of a sequel (which may come if SK ever finished Too Human).
