Lua was certainly an option and it's one I was even considering as an embedded scripting language back in the fall of 2014. I gave a talk in London where I asked for a show of hands about Lua vs. Javascript as the choice and the majority raised for JS.

One of the reasons we didn't go with that is because we didn't want a separate query vs. scripting experience like you have with SQL engines that embed programming languages. We wanted something that felt and looked seamless.