Rather an odd question really: trying to ask people to explain what is a matter of personal taste; and not just that but asking us to explain other people's personal taste preferences.
Firstly to the OP: maybe, just maybe KZ2 is a must have for PS3 owners who are fans of FPS games. I dabble in FPS but it is far from my preferred genre. So I might hire it to do the story mode one day, but that's about it. If I owned a 360 I'd have the same attitude to Halo. And given Halo "only" has ~10 million sales a MAJORITY of 360 owners probably have the same view, even in America Halo 3 is owned by a minority of 360 owners.
I think the respective reviews on this site give you the reason. Halo 3 got a 10 for value, KZ2 got a 9 (though KZ2 gets 9.1 overall and H3 8.8 overall; negligible difference at the end of the day). Many players will eventually give primacy to value over the other elements (as long as gameplay and presentation are of reasonable quality) because being able to play a game over and over with the same level of enjoyment as the first time is a big deal.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix
