After I joined ISC, I slightly explored the sister sites existing then. I joined the Kerala spider site(Spider Kerala.net).
But somehow I did not find it interesting as there was very less interaction. Then I did not think about it and forgot my user id and password too.
Subsequently a site Techulator was started with a good promotion by WM. Many members from ISC joined there as the remuneration at start was very good in comparison to ISC. As I was not at all tech savvy, I did not join it.(not sure whether I joined for taste and then discontinued). I am not a sportsperson and so not joined Sportsvale site.
Then came the Study village, and India travel blog. As they are all specific subject sites, I did not venture into them. When ISC was oriented or announced to be a full fledged educational site, I felt that I may not be able to sustaining ISC or long , as I am only a generalist and not specialist. But I am happy that still I am able to participate in ISC forum.
From my layman's guess I feel that either
(1) the sister sites were not really meant to be widely popular and were started to satisfy certain criteria or business needs,
(2) did not become successful due to lack of professional writers on the specific subject.
A specialist site cannot become successful with generalist amateurs.
I do not forget that there were some good WMs in some of the sister sites. Dwindling returns will keep good content writers away, and I guess that also happened. As ISC is an open general site(though favouring education) it will not suffer from brain drain, as new brains are always coming in.