In almost every organization I've been in, the reward for good work is more work.
Yes, but when that happens in a company, there is at least the prospect of a raise and more impact.
As a non-profit volunteer you get props and the good feeling of helping people.
It's one thing if you don't work in any area with operational requirements that change over the course of a year but if you do then you need to have reserve capacity for new workloads even if the people involved are good at work.