My Reason's
I think as soon as you walk up to a mac you notice how incredible the screen is. Even the original macs were known for their screen quality. I'm sure it is some technical stuff that has to do with the resolution of the lcd, but im a designer, not a computer engineer.
Also I really enjoy the interface, it is something about using an interface that focuses on asthetics that makes the whole design process better. Pulling up illustrator with OS X pin stripes instead of windows grey is a huge difference.
The dock is really nice to minimize work and then hide it so as not to clutter your work area.
I think it is easier to save and maintain large groups of files for one project.
The mac just looks beautiful, it honestly inspires as opposed to the utilitarian pc world.
Unlike other areas, there are no major design programs that are not on the mac. The mac may not have every business program, but you can be sure that it has every graphic program.
Last, and Most importantly. It just works. Everything is so integrated and easy to use. You don't have things crashing every five minutes. The mac takes all the hassle out of using a computer so you don't spend an hour out of your design time worrying about why your computer isn't working.