[mhd]

Seriously asking or just trolling? This statement could be made for almost any language. Why did we start with Python when there was Perl? Why did we start with Perl when we could use shell and awk? Why did we start writing something like awk when we could just have dedicated C programs? Why C when we had BCPL/Algol? Why Algol when we had... okay, granted, there's a limit.

Regarding the specifics: Lua is very simple, you can keep the whole language in your head easily, the table mechanism is very powerful, and the whole thing is easy to embed. Which is why it was used in commercial games and thus is industry/genre-relevant.