Desktop Linux has been getting better each year, though (at least on average). If it still hasn't crossed the threshold of being good enough for the mainstream, it's because their standards have also been increasing.
I've always thought the whole "year of the linux desktop" was a meme mostly kept alive by people who never have and would never use Linux anyway. People who actually use Linux don't think it's going to suddenly take over. On the other hand, most think it is actually a good system today.
Graphics, sound, built-in wireless, usb 3G modem, built-in webcam ... all of them worked out of the box.
The experience is great, although I'm still not recommending it to non-technical users ... once the Linux desktop is good enough, you still have the problem of apps running on top, and unfortunately the environment is not really friendly to proprietary software.
Pretty much the entire appeal of Mac is a) no viruses/spyware (because nobody writes viruses or spyware for Mac because its userbase is too insignificant to target) and b) big icons and prettier colors, making it "easier" to use.
Windows is an institution. It doesn't just give way once something becomes objectively superior; many systems have been superior to the their Microsoft contemporary and Windows didn't magically crumble. It's about installed base.