Two big things that contributed to the stagnation of EG Family (which I'm using as the umbrella for all the groups since 2011):
1) Not introducing enough new members. For such a large affiliation of groups, you need to bring in new faces to keep things fresh. You can't have members continue to leave without adding new ones. I understand that Flower didn't want new members, but all that did was speed up their decline.
2) Not adding new groups. They did introduce sub-units, but unfortunately ShuuKaren was short lived. I don't really count Dance Earth Party because it was still more of a solo project for Shizuka. It goes back to keeping things fresh. Why does Exile continue to add new groups while EG Family doesn't? I'm optimistic about the new group that will likely debut next year, but it's too little too late.
*I don't think it was as big of a problem, but putting Dream on the backburner was really stupid. There's no reason they couldn't have released a single every year.