Maybe there'd be plenty more devs if it wasn't written in a new, up and coming, difficult language to understand let alone master.
Sorry but this is a pretty weird criticism to have. It's like saying that a squirrel would be a better fish if it were a trout. A squirrel is a mammal, not a fish. Lemmy was intentionally written in Rust when the devs started the project. It's clear that it's in Rust by looking at any of the documentation. Yet this comes across as criticizing their project for what they've always said it was, while using said project to do so. Just a bit boggling.
If you like Java, contribute to Sublinks, if you like PHP, there's kbin or many other AP projects. Pick, use, and contribute to the project(s) that use languages and tech that you get excited about. Noone is forcing you to use someone written in Rust. No need to piss on other peoples' parades over language choice (it's not like they're using C# or Perl - kidding there, nothing wrong with Perl :P ).