IMHO, one of the biggest issues is that PCs have been "good enough" for some time now for everything but games, and even then they haven't been growing much.
I also see a major reason is that for a long time, PCs were the only way to do things. Want to see videos, read email, visit a website: You had to have a PC as smart phones/tablets sucked. So people had to get a PC. Really the average consumer didn't need it; they just needed something that would work and PCs were the only solution. These days tablets and smartphones fulfill what your average consumer needs. Add to that other features that PCs are not good at doing: ebook reader, jogging tracker, etc, its not surprising why many people are not getting PCs. For example, many parents were set up with PCs for a decade and barely used them. Most of the time we had to clean up a lot of infections. They both got tablets and use them all the time.