Marvelous Desiger is undoubtedly an incredible piece of software, but why so many great artists still haven't migrated to it, and still prefer doing fabrics in Zbrush? I believe that the convenience of creating everything in the same place is part of the answer, in addition to still being able to create incredible pieces of fabric directly inside Zbrush as well.

As for MD, in addition to the inconvenience of having to constantly exchange assets between it and Zbrush, there's also a considerable learning curve in MD. Yes, you can ganerate fabrics in MD in no time, but doing it with a professional look... that's a completely different story and requires a lot of knowledge. MD is a sensitive program, as fabric is a really delicate material, and therefore requires an expertise of equal value. The simulation can be VERY frustrating to master, with overlapping fabrics, collision problems, etc. 

I believe that fabric creation through Zbrush is more direct and simplified, and you can even use Alphas for the task, achieving great results just with it(and of course, with at least some basic fabric formation knowledge as well), and that's why I believe that so many artists continue to prefer Zbrush over MD. Perhaps if MD solves some of these mathers in order to facilitate exchanges between programs(like through some kind of bridge for example), the program could become more attractive.