The userbase of the PS3 has more diversity so there's always less people interested in one sort of games. PS3 has more casual players than the 360. In a long-term run it could prove beneficial for the PS3 but so far it's probably more of a hindrance... Sort of. I guess it improves their sales even now, but the question is how much.

Anyway, diversity is my answer.

"PS3 has more casual players than the 360"
O.K

My fanboy answer: Well PS3 owners buy the PS3 for the blu-ray and not gaming

My real answer: Lack of advertising (I see so many Wii, 360 commercials and I've seen one awesome Resistance 2 one), lower install base, 360 gamers may be more the "buy now at the 'sale price' for new release games because you know you'll play it later" type (even though sometimes later might be at a time when it's platinium or whatever).

Ask yourself why Gears is the successor to one of the most successful games this gen. Try to remember at one point, Gears was just a new franchise, and has been a powerhouse since day one for two reasons:

1. It's on the best console in regards to sales potential.

2. It had a great marketing campaign. The campaign was so good in fact that MS was able to tone it down for the sequel and still managed to outsell it's predecessor by a lot.

Here's answers to the others that all fit into my original points:

Fable II: There's nothing wrong with matching the original's sales. They were rather good.

Ninja Gaiden II: It's a niche game. It's exceptionally hard. It's audience is the informed gamer and it happened to score poorly in comparison to the first. It's not easily marketed because of it's violence. It's on the wrong console. Yup you heard right. NG should be on the PS3 also so it can leech off the dominant DMC4 userbase and the upcoming GOWIII fans.

Too Human: Microsoft decided not to market it. It's as simple as that. Without marketing it, the audience got pushed into the informed gamer and they know how badly it scored.

Banjo: Again, wrong console. MS doesn't have the audience to support Banjo-Kazooie. Marketing was good though, but there's no way this series can sell on the 360 right now. It belongs with Nintendo, and with Nintendo marketing.

MGS4: This game did fine. I will say that it could have been advertised more strongly by Sony, but it is what it is. I still don't get why they advertised for it heavily in Fall/Winter 2007 when it wasn't coming out until June. At least they bundled it though. If it's a matter of it not performing as well as past entries, it's just that PS3 is not as popular as PS2 or PSOne were at the time of release for the MGS games.

Resistance 2: It's a simple lack of marketing. FPS can sell on PS3. Look at Call of Duty. The difference is I've been seeing COD commercials every 5 minutes on TV and I have seen an R2 commercial maybe once. Sony is going to have to spend money to make money.

Little Big Planet: Wrong console. PS3 and cute sackboys and sackgirls do not mix. The Playstation brand's loyal base was built on racing, sports and action games. Secondly, the whole premise of the game is just not all that marketable or interesting to the every-man. By that, I mean the guy that isn't on the internet looking up details on games 24/7 like we all are.

Haze: It wasn't marketed. It underperformed critically. It's rather simple why it didn't perform well.

Motorstorm: That's the big WTF to me. Motorstorm 2 shouldn't have flopped. Racing games sell on Sony consoles. It has to be chalked up to a lack of marketing because I can't see what else it could be. This should have been advertised like there was no tomorrow.

Metal gear Solid 4 sold as well as could be hoped
Motorstorm 2 is just not a good game
LittleBigPlanet is selling extremely well for a new IP that doesn't involve shooting things

Probably becasue there are no games on the PS3 that are equivalent to Halo and GoW1&2. Those 3 games almost increase the 360's attach rate by 1.
Oh, and Sony's marketing is crap, I can't believe that they had more adverts in the UK last year than this year...

Sony marketing is crap?
At all 3 main gamingsite here,there is JOIN THE RESISTANCE.
At 1 of our main TV channel we have Sackboy standing behind screen in "coming next".
At our Big Brother there was mission about making your own Sackboy costume.
And one of our superstars is introducing at Tv his own LittleBigPlanet map.

Don't know about whole Tv marketing,I don't watch Tv that much nowdays.But Xbox 360 takes win there...With 189€ price tag in end.Thats what I remember.

But Gears of war 1 commercial was greates of all.I still remember it,and I hyped to my friends that if you get Xbox 360 just buy Gears of war 1...

NG2 and LBP are on there way to sell 1 million copies. MS had said NG2 had world wide already. 1 million copies is not a flop, but LBP was disappointing as i was supposed to be sony's big holiday title this year

@Onyxmeth: I don't know about the marketing. Sony is pushing really hard, but the lack of exclusives (in multiplatform games 360 has the advantage due to its price, which means it makes no sense to advertize multiplatform games).
To me the "crappy marketing" seems more like Sony not understanding to who they are marketing the product. The thing that Sony did wrong, was that PSP was more targeted to home console owners and pricing PS3 out of the consumers range, to who they wanted to market the product. And as a side note, the gamers Sony had in the previous gens, grew up and out of Playstation, so the marketing that worked earlier, doesn't work in the same fashion to same people anymore.