We want to know we affect our environments.
When we lose that power, we lose our motivation. Without motivation, people don’t participate. If your members believe their contributions will have no influence over the community, they wont make them.
You need to showcase the impact members have had over the community. Your members need to know they can have an impact over your community.
Your epic history will help. As will naming areas of the community after people, having regular awards, interviewing members about their contributions, mentioning the contributions in news reports, giving them personal attention, offering them opinion columns, giving power within the group, promoting them outside the community, mentioning their birthdays etc...
The more you showcase contributions, the more you will get.
Not every member will have an impact in your community, but every member should feel like they might.