The answer boils down to not only personal preference but for some cases it's easier and more orthogonal to use. Even if that _wasn't true_ and many people are going to shit on me for saying it _is_ true, but even if it wasn't: the perception is that it's easier to use, more orthogonal and made with premium materials.
Aside from the pro-sumer market, (which is still hit and miss) the build quality of laptops and battery life is sub-par in Windows-land. Add to the fact that unless you have a Dell support contract you're going to be fighting your laptop vendor over random things potentially and suddenly you're not looking at an appealing platform.
This is _especially_ true if money is literally no object as is the case for screen writers who are probably the most well compensated people on the planet (as roughly 30% of a movie budget goes to the screen writer(s)).