Quite simply, I think that the whole "add your note here" had quite an alluring factor to Flickr. I am guessing that as more and more people started hosting pictures on flickr, they found all the useful tools there to be very suited to Space MOCs. the Group function seems to be something suited to Space MOC's, considering all the groups out there (NBT, Starfighters, Vic Vipers, Blacktron, M-tron, VOAT, Classic Space, to name a few)
The old CSF was an old style forum, in a sad state of disrepair, (if I recall correctly) as well, so I expect all the high end builders 'left for greener pastures' so to speak.
The first theory of Ketchup: With enough Ketchup, everything tastes the same.