I'm not convinced that that is actually true (that it will bring more contributors, that is). We'll see, but we have yet to see it.
Rust is an extremely hyped language online these days (just check the size of !rust and !java for example). If we talk about contributors, I don't think it's a bad choice. Yes, more people right now know Java (apparently not on programming.dev though? 😅), but probably a lot more people are interested in learning Rust than learning Java (Stack overflows survey suggests so). In the (not close, to be fair) future, there could easily be more Rust devs than Java devs.
Besides, in my humble opinion as a professional software engineer, Rust is just a better language than Java. It has native speed and much lower memory usage, but more importantly, it's more reliable and robust than Java (better/more complete error handling essentially). This shouldn't be surprising - Rust is a much newer language that has been able to take all the good stuff from many different languages from the past.