Fediverse

It's just how the fediverse works in general. Content is sent to recipients, similar to email. Which includes your followers. But if you're not a recipient, you never receive the content.

On Lemmy it's less of an issue for individual users because communities act as relays. They take posts and comments made to them and forward them to all their followers. It is however a problem for communities themselves.

Lemmy does have Lemmy Federate however, which helps improve the reach of its communities. You just enter your community and it'll use puppet accounts on various instances, both big and small, to subscribe to it, allowing you to reach the world even from a self-hosted instance. I don't think the Mastodon side has something like that for users.