Price, apps, market share, corporate domination.
Thing is, TCO (total cost of operation) over the life of your computer, start adding up those geek shop visits, having to BUY the majority of your apps (if you want well supported pro quality ones), the risk to your data of malware, the risk to your data of being hacked, there's a case to be made that, if you keep your computer for any length of time, the Apple computers start to end up not as expensive as initial cost makes them seem.
If you are fairly geeky yourself, and know how to keep a PC system humming along at its best, if you know how to edit the register, control the background processes, and all the other important tasks to keep your PC worry free and running, you may be good to go. But OTOH, how many TV adds you seen for services and software (none of which are cheap) to speed your Mac up?
None. Doesn't need them.
Mac's are for people that would rather concentrate on the TASK, than the tool that performs the task.