Inertia?
Don't forget that Mac PCs are almost universally "prettier" than other PCs. Design and graphics types like a pretty machine. That has a LOT to do with it.
Of course inertia/history plays a part and don't forget ignorance. Many Mac users in these fields quote that you can't run their apps on PCs or they don't run as well or all sorts of other things. None of them will have tried.