I would guess that the reason is that the CPU was not the bottleneck for normal gameplay. Since I guess you still have the same GPU it's probable that the framerate is limited by that. Also, it might be that the graphics engine don't use mutiple cores effectively, and since the individual cores on your new CPU is only slightly faster, the difference might not be very noticable.
If you have any way of monitoring the CPU load on the individual cores during gameplay, you could see how much the game uses.
