Programming goes through fads where people will claim X can solve every problem. Eventually people realise a languages strengths/weaknesses and communities form.
Rust is the current fad language, its developed a strong following in C/C++ communities but they have nothing to do with middleware (the role Lemmy is using Rust).
It means lemmy devs will have to build everything themselves (instead of focussing on lemmy) and the pool of contributor's will remain small.