They marry themselves to it because it is so easy to focus on Apple; the style of the products, the exclusiveness of the software/hardware, and the size of the company/market share all make Apple a solitary, unique brand with a style that is easily accepted and enjoyed by a large amount of people. Think of other brands of MP3 players, phones, and computers – all of them are made by several different companies that utilize all sorts of styles and functions.
MP3 Players all look different from brand to brand. Apple pioneered the field anyway, so that’s why Apple dominates MP3 players. When it comes to phones, of the major brands and, more importantly, operating systems, Apple is centralized in it’s single product while OSes like Android and Windows are all decentralized over several models produced my several companies. When it comes to computers, while most competition comes from Windows, the OS is available on a massive variety of computer styles, types, companies, and looks, not a small selection of similar looking and acting computers.
I think the key that Apple has is that it’s brand is so centralized – if you use Apple products you have either all Apple or none at all; you have both software and hardware or neither. There is minimal variety and difference – your computers, MP3s, and Phones are all similar. Once the brand is centralized and visible, it’s just a matter of finding the people who like the unified style to create your cult followers, and because those people who like Apple like very similar products, you get the “cult” sense.