Equally deep, though less influential but more permanent is the pre-existing PC infrastructure. Apple, being the only purveyor of Macintoshes, doesn't have to worry about anyone else when they change designs. This is why the company has been able to smoothly transition from the original Motorola to Motorola (later IBM) PowerPC designs, and make the drastic leap from the old MacOS to the new one with much less effort than Microsoft had to expend to convince Windows users to upgrade to Windows 95.
More importantly, Apple controls both hardware and software. While this presumably lends Apple advantages similar to those of a console maker, it's actually more freeing for the company than locking it into a single design. For example, since Apple does its own hardware and software design, when they made the Mac Mini, they could do any proprietary software in-house. While this is true of most companies, few are as adept at software and hardware design as Apple is. Apple's software engineers deal with Apple hardware from the ground up. Dell's or HP's programmers work only with Windows, and their contributions are relatively minor to the system as a whole. Microsoft has little to say with the hardware process. NVIDIA, ATI or Intel are focused on isolated bits of hardware and the supporting drivers, not systems architecture. Apple has almost complete control of the entire product.
In the PC market, there are dozens of competitors. Coming out with a completely new product can be done in two ways: in tandem with enough of your competitors, after years of negotiating a standard, or doing it alone with a "unique" design and facing hordes of copycats. By being the sole Macintosh creator and being vertically integrated at the same time, Apple can quickly design new products. It is neither dependent on co-operation with competitors to establish a standard, nor does the company have to worry about competition within its core market. They have to fight for swing users, but the Mac hardcore will always be there.
That too is another advantage of Apple. The Macintosh hardcore are not only unwilling to buy any other computers, they're declined a choice. Whereas PC makers have to constantly compete amongst themselves and this, due to the culture among PC users, focuses on performance and prices, Apple just has to avoid angering their own market with excessive fleecing. The brand loyalty to a Dell, Gateway or HP is much, much lower than it is to a Mac.
Finally, there's an entire infrastructure dependent upon Apple itself. Due to the weight of historical momentum, cultural appeal and simple familiarity with the platform, graphics and sound professionals stick with Apple products. Since Macintoshes were the first personal computers with relatively advanced graphics and sound at an affordable price, they were natural candidates for professional software. Since then, the trend has merely perpetuated itself - it matters not that a PC may be faster at doing something, since all graphics and sound professionals own and work on Macs anyway, software companies make their editing tools for Macs. Even the appearance of quality tools on the PC, such as Adobe's line of products, hasn't made much of a dent in the professional Mac market. Indeed, if anything, it's only created a larger pool of potential Mac users since they have been trained on the same software for the PC, while all the jobs are actually offered for Mac users.