Historically speaking, we've found that OS categories generally don't do very well (besides Windows, of course). Tom's Hardware is primarily a hardware site. Our editorial articles are centered around hardware and the audience we attract is hardware-oriented, so our forums tend to reflect that.
If there is a significant interest within the community for this, it is definitely something we are willing to consider. However, most of our non-hardware forum categories see fewer than 50 new threads per month which indicates to us that there just isn't a great demand for them.