Lack of awareness
The people I know who have acquired macs mostly did so because they liked the interface and because it just worked. They have absolutely no intention of opening it up, expanding it, upgrading it bit by bit.
If it breaks, they take it to me (alas) and then to the nearest Apple store. Furthermore, if past experience is anything to go by, they will use it for about 5 years until it breaks down or they are given a new one.
They are well aware that PCs are cheaper and could have gotten one for less.
Apple's success in the last 10 years has been due to iPods, iPads and iPhones. Mac sales are higher than before, but not that much higher. Apple designed and marketed devices that people wanted to have and liked to use. They were at the pricier end of normal and they were considered items worth spending a little bit more on. The iTunes store and massive number of non-buggy apps made for a very comfortable experience.