The pressures are largely different. Apple has, for years, been a company that survives largely by doing things with style. One may or may not like theirs; but one has to admit that they know that they depend on it and certainly work hard at maintaining it. Having a decent size pool of talent that is really, really touchy about style and feel is somewhat unusual in the business. And, in the end, they get, more or less, what their pressures demand. Like them or loath them, macs are tightly integrated and distinctive.
Linux, on the other hand, is moved primarily by a mixture of geeks and corporations that stand to gain if Microsoft and/or Intel lose. The both the geeks and the companies like powerful features, great stability, and low price. As one notes, Linux has all these things. The geeks generally don't much care about cute interfaces(note, they quite like elegant interfaces, like good package managers, and interfaces that can do cool things, like all the network transparent protocols for practically everything) and stuff, either actively in the "GUI is for l0sers" sense or in the passive "I'd rather be hacking the kernel or the Gibson than writing some silly eye-candy" sense. The corporations, generally, want some level of interface; but are more looking to be good enough with respect to Windows than to be style kings. As a result, Linux is decent but not hugely exciting in this area.
Also a factor is hardware control. Apple knows precisely what it is going to be running, and can plan accordingly. MS has the advantage that most manufacturers write drivers to its specifications, and it is able to sign various restrictive contracts in order to distribute drivers. Linux has neither advantage(except in specific instances e.g. an IBM Linux-on-Power or something of that sort is going to have its drivers in order). It runs surprisingly well on all kinds of wacky stuff; but most manufacturers don't bother to include linux drivers or even make documentation available, and only some stuff can be reverse engineered. Drivers are perhaps the thing that makes Linux installs most difficult.
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