In 2000, the only feasible language really was C. C++ suffered from severe portability issues, which have only arguably gone away in the past few years (that said, you still do see systems with pre-libstdc++-4.9, which is still a headache for portability). Managed languages generally suffer embedability problems, and still do to this day.

The conclusion that C remains the best language is a lot harder to support. Certainly, and especially with the level of infrastructure that SQLite developed to harden its implementation, the alternatives are not so much better as to be worth the cost of migration.