Why are most repositories maintained by only a few contributors?

Why are most repositories maintained by only a few contributors? And not active.
The most frequently updated repository, aeternity, has only 47 contributors, while other repositories generally have less than 10 contributors
As a contrast,
Bitcoin, 699 contributors;
Ethereum, with 466 contributors;
Tron, 127 contributors;
Dash, 390 contributors;
EOS, 200 contributors, etc.