I'll take a stab at it since I used to work for a print shop way back when. The reason they still use predominantly macs is because.... everyone mostly still does. Nearly all magazines and newspapers are still done on macs. Strange huh?
There are 4 reasons for this:
1. Color Correction/management
Microsoft never until very recentlyquite figured out how to make designers happy with the output and the color they'd get. I can remember all the stuff we'd get and send to the printer only to have the greens turn out pink and the blues turn out greenish-yellow. Yeah that's an extreme example, a lot of stuff did come out "almost right" but almost right doesn't cut it in the pro printing biz. If the colors are not 100% exactly matching the pantone swatches the client picks out, its a redo. That's expensive.
2. WYSIWYG/Font management
A lot of problems with early PC software was related to the fact that ms/hp chose pcl and the printing community chose postscript. HP printers have peculiar behaviors-- Pick an hp printer with "built-in" fonts. Amazingly they suddenly show up in your fonts menu and you use them. Naturally when you give your document to someone else with a different printer, the helvetica font on that printer is slightly different than the helvetica font on that other printer. Consequently all the lines don't match up anymore and the whole document is ruined. You just couldn't give printer fonts to other people-- you were stuck with the "built in font" and that was that. The other thing was WYSIWYG issues. You could have the document look great in the "print preview" but when you go to print it out, it looks different-- for example I've seen a lot of one page documents that when printed, ended up with another line on a second page for no apparent reason other than maybe the font issue I pointed out earlier.
Macs do a few things very well and printing/color management is one of them.
3. Applescript workflows-- apple made everything in the os scriptable and many of the apps vendors did too quite a while ago. So, a lot of newspapers have these scripts that suck stuff out of databases, clean them up in word, match the item in pshop, crop all the photos, send it off to the classified ad section, lay it out all pretty in quark, then force it out for preview all in one script. You could probably do this with wsh or vb but the newspaper people already have it all set up in applescript. Good luck getting them to change.
4. quark - everyone who published real stuff uses quark. Quark uses Xtensions. Many of these used to be mac only. So there you have it.
Anyway, these are 4 reasons. I'm not sure any of them are completely valid anymore. The first two have been greatly improved with opentype and ms eventually buying the same color matching technology that apple uses. As for the applescript and quark, well there is quark for the pc but I have no idea regarding the Xtensions.
The big unknown is how designers will move to OS X. Apple has pretty much abandoned the print crowd for the dv crowd these days. They've recently purchased a lot of high end video and 3d companies which create pc products (ie:shake). You can assume that the pc versions will be killed off as they killed the pc version of final cut pro (purchased from macromedia).