[lneto]

Please refer to Section 3 from [1], this was extensively discussed there.

[1] https://www.netbsd.org/~lneto/dls14.pdf

However, giving you a short answer, we use Lua mainly because it's easy both to extend and to embed. It's implemented as an almost "freestanding" C library. Moreover, it has a considerable small footprint (~250 KB) in comparison to other scripting languages (e.g., Python has a few MB). Moreover, Lua has automatic memory management, fully isolated execution states, protected calls, among other features.