Originally it was Lightroom that was the primary barrier, but that came to Windows just over a year ago. It still doesn't run as well on Windows though, has frequent problems, and doesn't integrate as well with other hardware you're using. Macs just work when you set up your cameras, lights and backups straight to lightroom for post-processing before editing. And a photographer probably wants to integrate with a tablet too because it's just easier in a studio environment, which can sync perfectly with the main machine running lightroom on both the tablet and the machine at once.

In short, for anyone that gets serious about photography it's about the workflow streamlining and Apple have that nailed because they tie together absolutely everything. It is a much bigger faff to get a similar workflow running smoothly on Windows. Not to say it's impossible but you're going to be using all sorts of workarounds.