Github stars is not a good metric, firstly because KBin is hosted on codeberg but mainly because a healthy project has lots of unique contributors and regular updates/enhancements.
KBin has 79 open Pull Requests, while Lemmy has 29. From a visual check PR's seem to be older than 2 weeks. Its hard to say one is "healthier" than the other, without scraping information into a spreadsheet.
Secondly Rust is new and has a lot of hype surrounding it, as a result you get a lot of people using it on random projects.
Languages have strengths and weaknesses and developer ecosystems build on the strengths.
For example if I was writing a web application with a database backend I would choose C#, Java or Node.js because there are loads of libraries, tools and frameworks to make it really easy.
Rust is gaining a lot of adoption by embedded system users (replacing C mostly). Lemmy is the only Rust based web server project I am aware of. Which means the level of work to do anything and to keep it updated falls on the Lemmy devs rather than spread out amongst a larger community.
Everyone loves to insult PHP but it has a niche in webservers and won't disappear anytime soon. KBin effort will thus be spent on KBin.