Until a software system - application or OS - provides overwhelmingly superior advantages for a user it's not worth it for them to spend the time required to learn a new system.
Thats always the biggest factor. For most users the most important thing to do is what they want to do, not learn a new piece of software that is 'better' in termes of a vague philosophy that someone else is trying to persade them of.
The MAC/PC/Linux argument is in a lot of ways irrelevent to the user. Any idiot can face a crisis - it's day to day living that wears you out.
