Because BIS never liked to optimize. Operation Flashpoint had huge REAL requirements (what is written as requirements in official documents is always a joke). Also arma 1 and 2 had huge requirements, greater than any other games at that time, and the releases become more frequent. So when i bought a PC that runs these games nicely, the next part came out, with even bigger requirements... so arount Arma 2 i give up with this sequel.
I'm not gonna play on low FPS, nor i want to spend money on a new pc just because this game(while every other games run nicely).
BIS programmers are just lazy. They said at the release of arma2, when people asked why don't they use directx 10.1 (which has a lot of good features for an open world game like this), but they said "we don't see enough improvements in it". They rather say, "the user will buy a better PC so we shouldn't need to optimize"
By the way, if arma 3 still uses directx 9, that eats the most of the cpu! Every open world game that uses old 3d techniques is limited by the single core performance of the CPU, as directx9 can use a single core to generate the draw calls (every object in game, needs a draw call to be sent to gpu in every frame).
the usage of nvidia Physix will make it worse. it's a pile of junk, with a lots of restrictions. It's usually used by game developers for money (nvidia pays for developers to use their **** so people will buy nvidia video cards too... while in reality 98% of the physix effects are run on CPU and not on GPU. The only difference is, when it detects nvidia gpu, the physics using all of the CPU cores, if it isn't detects nvidia gpu, physics is restricted to 1 cpu core).