I think you are wrong, of course Sony wants every title to sell well, but they have a wide spread of titles that fits different niches, together they cover a lot of ground, and I am saying that KZ is not meant to fit the same wide niche as Halo. The niche of Gears of Wars may be more comparable, it´s a lot about how the title is marketed, but I am no going to try to analyze that.
By targeting a more narrow niche you´ll have a less potential buyers by definition and that is probably why those titles has not reached Halos number. Not saying those niches must be small, they may still be large chunks.
If you still think KZ was marketed to sell to the same wide niche as Halo3, we are just of different opinions and that is OK by me.
You can tell how broad of a market a publisher/console manufacturer is targetting based on the level of investment they undertake for a franchise.
Sony not only paid for the development but they acquired GG in the process. GG output at that point was Killzone and ShellShock of which Edios retained the IP (ShellShock 2 was published early this year by Eidos and developed by Rebellion). You don't pour that type of money into a franchise with the intent of targetting a small niche market as neither have spent much time within the mainstream or casual price ranges.
The 360 and PS3 have for the most part targeted the mature market primarily. You only have to look at their top selling games and see that a broad swath of their market look like the perfect target audience for KZ2. Furthermore looking at the retail price history of both the 360 and PS3 would only help solidify that argument.
When your top selling list consists of titles like these, how can you say KZ2 was intended for rather limited hardcore niche market within the PS3 userbase.
Gran Turismo 5 Prologue
MotorStorm
Metal Gear Solid 4: Guns of the Patriots
Grand Theft Auto IV
Uncharted 2: Among Thieves
Uncharted: Drake's Fortune
Resistance: Fall of Man
Call of Duty: Modern Warfare
You might be very much right in accessing that KZ2 was too hardcore to accomplish blockbusting sales. I would say in that case, Sony misread the market and that Sony's market presents a level of complexity that can't be described superficially. Looking at the level of investment, the appetite of Sony's userbase and the PS3 retail price history, I see a product that Sony intended for a broad part of the PS3 userbase.