You want a member to embrace the community identity.
You want them to mentally commit to that community.
They wont do that if they think the community is struggling.
Why waste their time, effort, and sacrifice a part of their own identity for the group if the group might fail? That will just hurt their own self-concept.
This is partly why you need to concentrate activity in as a few as places as possible. It's also why you need to identify your quick wins.
The perception of momentum and success, is as important as success itself.
This means celebrating the minor things in the early stages, regularly introducing newcomers that join the group, writing content about upcoming events and activities, initiating regular discussions and ensuring the popular ones remain as sticky threads for a week.
If your community isn't successful, it's a good idea to make it appear like it will be.