I think its for two reasons.

1. They make games which often don't appeal to the broadest of audiences. They know how to hype it to the hardcore gamers but they don't seem to know how to make games which your general audience will pick up. Marketing and advertising is fine, but it doesn't help if the word on the street is against you or some other game is more interesting than yours.

2. They almost always screw up their game in a subtle way. They have talented developers but years of success has made them lazy. They seem to be doing their best to screw up the mantra that 1st party developers are better because the third party developers are hungrier, they sieze opportunity and they make more with far lower budgets than Sony throws around to its developers. Their best success recently in Little Big Planet is ironic in that it comes from a team whose core were ex MGS (Lionhead) developers.

I think:

1. Sony´s economic situation hasn´t allowed the the aggresive marketing of games we are used to from past generations thats why it´s lagging.

2. Sony´s fanbase knows that they have basicly the same games as the 360 and when a game is advertised better for the 360 the sony fanbase knows this game will be on PS3 and thats why multi plats sell good, while the exclusive are really only known to the ultra hardcore sony fans and add that not that many commercials are shown for Sony games then you get the results we have.

Ok. 

By the way, the rest of my post answered your question.

Sony is improving.

But it's quite simple..

There ads have been pretty bad mostly..

they've started to become better..

but look at killzone 2. It had a random ass ad that can't have sparked any interest (the one on tv.. the bullet one) in america.. it had a regular ad in europe though.. and it had a awesome trailer that could also have worked as an ad. But they chose just 1 bullet flying over and hitting that random dudes head :\

Infamous has had great commercials in america and it's done well so far for a new ip. Lets see what it ends up on.. however, i haven't seen 1 single ad for it where i live.. not even a poster on launch day.

I don't get what you mean. The demographics are simply more diverse. This means that on Xbox, the shooter exclusives sell much better, and also the MP shooters sell much better on Xbox. On PS3, the exclusives are more diverse so each games sells less, but still you tend to see higher percentage sales on PS3 outside of shooters

see my reply, i think its as easy as that.

You know theres a school of thought which believes that its not the quality of the advertisement which sends the signal but the fact that the advertisement exists. See if you have two games, a good game and a bad game if you have say $10M to spend, if you spend $10M on the bad game you might get $8M back whilst the good game would give you $16M back. So a company would not generally advertise unless they believe their product to be good/great.

"I have to say a yes...

Top multiplats sell in fairly equal percentages (considering install bases) on both 360 and PS3 - FPS, Driving, Sports and Fighting titles... they all sell in very good numbers.

Decent exclusives sell in bucket loads on the 360

Decent exclusives seem to underperform on the PS3

I dont buy the pious "diverse demographic" otherwise the multiplats would also suffer.

KZ2, Res 2 and InFamous have suffered partuclarly from this it seems, in the case of KZ2 and Infamous very good games, that were well received critically (clearly) and yet not exactly going gangbusters in sales.

KZ2 sales have stalled at 1.7m way below expectations, even of the level headed sony fans here, and InFamous needs to pick up or could see less than 1m. (of course these figures dont count any bundling that could take place)

Sony are relying on brand power and are letting their excluisive developers down, they need to start marketing these games much better.

Thoughts?"

Well, if you look at games like bioshock (which at the time was a console exclusive) and dead or alive 4, you will see that they didn't do as well in sales compared to other well known titles like halo.  

Also, with the playstation 3's high price, you won't see games selling like hotcakes until sony brings the cost down to a price everybody is comfortable with.

I think the problem lies somewhere else then in marketing, Sony's exclusives this generation are great, but they arent big renowned franchises like the multiplats that have sold well.

RE5, GTA IV etc. etc. , they are big games but they appeal mainly to the most hardcore of the users, Killzone, Resistance , Infamous , Uncharted and Little Big Planet mean less to average console user than those big games that have been around for ages, and they failed to make them big like Gears , Assasins Creed or Wii series. In contrast MGS is a big franchise and it sold extremely well for an exclusive

And with limited userbase even big advertising push cant help it, Last Gen Sony had the biggest exclusives now there offerings are unable to adress the majority

It's quite obviously just a symptom of a smaller install base. When you have a smaller install base it becomes very hard to sell anything that isn't an established brand.

As you can see from the top PS3 sellers, the best selling games are pretty much all multiplat, established franchises. The PS3 lacks a large install base so they lack diversity. Also notice how the PS3 has had its most success in Europe, therefore European games sell better. Games like Assassins Creed and Pro Evo.

You guys put way too much emphasis on marketing.