I think beyond the historical reasons why C was the best choice when SQLite was being developed, or the advantages it has today, there's also just no reason to rewrite SQLite in another language.
We don't have to have one implementation of a lightweight SQL database. You can go out right now and start your own implementation in Rust or C++ or Go or Lisp or whatever you like! You can even make compatible APIs for it so that it can be a drop-in replacement for SQLite! No one can stop you! You don't need permission!
But why would we want to throw away the perfectly good C implementation, and why would we expect the C experts who have been carefully maintaining SQLite for a quarter century to be the ones to learn a new language and start over?
We don't have to have one implementation of a lightweight SQL database. You can go out right now and start your own implementation in Rust or C++ or Go or Lisp or whatever you like! You can even make compatible APIs for it so that it can be a drop-in replacement for SQLite! No one can stop you! You don't need permission!
But why would we want to throw away the perfectly good C implementation, and why would we expect the C experts who have been carefully maintaining SQLite for a quarter century to be the ones to learn a new language and start over?