There doesn't seem to be a strong reason to pick one drug over another. You are right - there often seems to be no real strong reason to choose one chemo over another, especially after a first one. This is one of the many things that makes OvCa so hard to deal with.
Bottom line is there is no real cure for everyone.
There are so many different types of ovarian cancer, like many cancers. What works for one patient can be less effective or just useless for another. As inexact as it is, it is all we have for a starting point, and so we go with what works best for most. Along the way, we have tests, we note that CA125 number, and we adjust or change depending on results. We also have scans, etc.