Apple is becoming an 8000 pound gorilla because nobody else can design a user interface! Windows is crap committee-ware, and open source UIs insist on imitating Windows (look at Gnome, KDE) which means they're imitation crap.
There are other aspects to Apple that matter too. Their code performs well and doesn't tend to break. There's a polish that Apple's code has that nobody else puts on their product.
Their hardware is decent (not excellent, but decent) on the quality factor too, but it's also well-designed just like their UIs are. The biggest feature of Apple's hardware is what it doesn't have: loads of useless buttons (stupid blue ThinkVantage button, I'm looking at you!), connectors you never use, etc.
Note that Linux rules the server space for the same reason. Open source UIs are crap, but under the hood the open source ecosystem obviously cares. So in markets where under the hood matters most, Linux and other OSS operating systems are the dominant players.
The moral of the story?
Stop listening to the idiots that tell you that the way to design software is to slap it together and focus exclusively on marketing. This only works if your market is not crowded. This worked for Microsoft in the 80s and 90s because there weren't many affordable alternatives.
People actually do care about quality.