Now... not many are buying home PCs.
Because most people already have a good desktop or laptop PC at home. PCs do not change as much as they used to, so you do not need to buy a new one (or even upgrade your current one) every year like you used to (unless you play games and really want to have as high FPS as possible). A PC now lasts for many years for common tasks like web browsing.
On the other hand, tablets and phones change a lot, while the hardware may not change as much (or rather, as noticeably), you cannot most of the time upgrade to a newer software version on your old device (like you can upgrade Windows Vista to 7 for example).
Of all people I personally know, the majority have a PC at home (those that don't are usually older than 70 years - didn't need or use a PC all their lives, do not care to start now). Sure, some also have tablets or smartphones.
The measurement of sales for something that most people already have is weird for me. Sure, you can measure sales of some new technology or limited-use things to see how they are doing, but to say that, for example, radio is dead because people are not buying new radios as much as they used to even though most people have a radio and use it (at least in the car) is a bit wrong. Most people already have a radio, a TV and a PC at home, just because sales are dropping may not indicate that the technology is dying, it may be that the devices people already have are still good enough.