Because part of R2 still retain R1 design elements. It is not CoD. Adopting some of CoD4 elements may require more surgery.
R2 has MUCH more monster types than R1. Many are unbalanced against the player. The weapon wheel makes the fight more apt and interesting.
R1 is more arcady/heroic. People expect to run out in the open, strafe fire and kill everyone else. R2 requires cover constantly due to the short health bar. Does KZ2 have very short health bar ? (I have no idea)
R1 is also unique because of other design choices like having a distinct Chimera player type in MP.
I believe it's how the elements are packaged together. Despite the above issues in SP, R2 co-op is a winner because different player roles/weapons made up for the wheel and encourages us to co-operate more. You also get healed by the Medics relatively quickly. R2 co-op is arcady/heroic against smarter and smarter AI. You are constantly on the move, changing strategy to try to flank the AI (because they have far greater numbers). That's why I am still playing R2. ^_^
Here's where I disagree with you:
o The Co-op isn't arcadey or heroic. It's the same as Single Player in many ways. Cover is almost always required, because if you run out without cover, you will ultimately die. Medics can't simply run into a mob of enemies and kill them, because they will likely die. The reason they survive is because the Soldiers are providing some cover with their shield. Spec Ops tend to generally be around Soldiers or Cover, tossing out ammo. The difference in feel is because of the class system, but at it's core, Co-Op has a lot of the same elements of the SP, in that cover is required and you can't just pray and spray.
o The unique Chimeran class in R1 was removed because it wasn't balanced, period. The fact that you had to cool down from rage really didn't mean much when you're playing with an organized team who can alternate who is / isn't on rage. It simply wasn't balanced, and the decision to remove it was the
right one. Balance is far more important than being unique. Even then, it is still unique because you still have the hybrid vision (without the ridiculous damage boost).
o R1 rarely felt like a struggle because you were a walking tank. Honestly, R1 was a breeze on any difficulty but Super Human, and you could just use your weapons to make up for your health. R1 actually had a significantly HARDER health system, requiring players to stay in cover more becuase you only regained a part of the health, and you were often left back tracking to find health kits. R2 eleminates that problem and gives you full health, but also removes your weapons so you don't feel like a walking tank. Part of the problem with R1 was the amount of ammo / weapons you could hold, which made you FEEL like you had a lot of health, etc, because you could dish out a large amount of damage via alternate fires quickly without leaving cover often, or getting too close.
o There is not one enemy type that I feel is unbalanced. Except maybe the furies. I've already discussed my thoughts on the Chameleons, and they are certainly not that bad, especially if you've been through the game once. It's funny, because everyone was complaining about how "stupid" they were since they always game from the "same direction". Now they're complaining because apparently they were wrong, and it's exactly what the hell they were asking for. Absolutely ridiculous. The fact that headshots in this game are FAR more lethal than R1 makes it loads easier. I can mob out a great amount of chimera with a Bulleye MKII or Carbine from decent distance by aiming for the head, you can even take out ravegrs with a few hits to the head with those weapons, or one shot with the Marksman.
I don't know, I disagree with so many points, I really feel that a lot of the complaints stem from hive mind. There are many more high profile games that had far more problems with design (CoD4 comes to mind) but didn't receive half of this 'hate' from forum gamers (note: casual gamers love it, and I'm positive they far outweigh the internet casuals who spend their time on forums like these).
I apologize if any of this seems brash or rude, you know I've got nothing against you
Or anyone else, I simply don't feel that a lot of the complaints are justified when they are passed over in other high profile games. I could point out flaws in many of those titles, but it simply wouldn't be right, since I still enjoyed many of those games as well.