A couple reasons come to mind:

1 - the social groups are not present anywhere on the homepage, only in the menus.

2 - the groups don't seem to offer any special features that the forums don't already have.

3 - Userbase: there are hundreds of users, but only a small group that are active on a daily/weekly basis.

4 -my reason: I don't use email alerts, I check the forums daily but forget the social groups are even there.