Question Author
Ok. Again answering my own question, back in the 80s, the only usable hardware for page make-up was QuarkXpress (and in those days you could only run it on Macs). Similarly, Photoshop was slow and horrid, but only Macs could even start it.
So we used Macs.
These days many fifty-quid word-processing type programs do the same jobs as Quark (well, not quite as comprehensively, possibly not as well), so still Apple/Quark gets the job. At least Photoshop is now widely available as a Microsoft program.
I think it's largely inertia, the trade not changing to Microsoft products.
But why 'ordinary' people should use Macs (unless it is a fashion statement, i.e. "I can afford two grand-plus for a laptop and you can't so boo") I don't know.
A