Apple do it better
I'm no fan of Apple per se, but they do things better.
MP3 players, slate PC's, touch screen phones and PDA's all existed before the i-Pod, i-Pad, i-Phone etc.
None of Apple's competitors offered a particularly easy, comfortable, smooth and responsive experience for the user; because none of them wanted to.
None of them wanted to provide an integrated and controlled experience, and none of them particularly cared if anything worked properly.
I'm not saying that Apple hardware is more powerful when comparing devices (in 'apple for apple' comparisons), because it isn't.
What matters is that your wife, your sister, your girlfriend etc can view it as a fashionable and expensive item; that it works with minimal attention to an instruction book, and that it does what they want it to.
Plus - by offering all this stuff in a heavily controlled sandbox environment, they've managed to get all the content developers and content owners to jump on board and offer everything they can (if they can get Apple's permission first).
These are all reasons why if people want an MP3 player, or a new phone, or a slate PC, they will strongly consider Apple before any other brand.
They would probably look at the alternatives and see a lack of decent available content, or a complicated user experience, or stuff that just doesn't work or is too damned complicated to use.
How many times have you heard somebody say 'I should have gotten an i-Phone'?