For the longest time, all the good software was only available on the Macintosh, combined with post script support, color managment, ect, the platform got a good hold on the market in the early-mid 90's, and many people never switched back, even though now many of the tools and features available on different platforms are as powerful, if not more so than they are on the Mac.
Photoshop is a great example. Back in the PPC days, PS flew compared to x86 systems, but when Apple switched to x86, that advantage went away, and is now actually the reverse. Photoshop CS4 runs faster under a Vista x64 machine than it does on OS X, on the same hardware.
And this of course has lead some people away from the platform. Some of this has to do with the availability of software on competing platforms, some of it has to do with many people feeling abandoned by Apple because of their recient hardware and software decisions (glossy displays, dropping firewire for a while, really long periods between software updates, dropping out of NAB, hardware becoming more consumer-focused, ect), and because often times the hardware is less expensive, and the support is better on the "other side".
I'm not saying that Apple doesn't still have a large share in the "creative professional" market, but it is definitly shrinking.