There was a blog on Battlefield.com about why the PC version of BC2's Frostbite Engine was overly CPU intensive in the final version of BC2. The blog went in depth explaining the Sound Engine for BC2 and how it ran into only a single hardware thread on the CPU and not in software like previous Battlefield games in the past on PC.
That, coupled with the fact that the Engine could not handle more players than 32 with all the sound engine was processing along with the destructibility of the levels without having serious performance issues. This is good reasoning to why PC was limited to 32 in BC2.