The language is well designed, with orthogonal features that complement each other. Coroutines and meta-tables are easy to understand and still powerful way to organize and control code base.
You can actually learn it in a week, and hold all of the language in your head. A very valuable thing in my opinion. The code is very readable to beginners (unlike Perl, Haskell or APL).
The interpreter is incredibly fast. Even non-JIT implementation is surprisingly fast.
The LuaJIT FFI is so convenient that the difference between C/C++ libraries and Lua libraries almost disappear.
There are cross-platform hosting frameworks that allow you to write and execute dynamic interpreted code on otherwise locked down platforms (Android, Switch, TVs...).
I would say Lua lacks tooling and better organization of community. Biggest Lua communities were built around products that embed Lua like WoW, Roblox, Corona (now Solar2D).
I think that's because there aren't many things it's really good at. It excells at a niche – as a light-weight, easily embeddable scripting language – but is mediocre as a general-purpose programming language, because it's standard library is extremely spartan and lacks so many things you need on a day-to-day basis.