This interview addresses that:

http://www.techworld.com.au/article/260022/-z_programming_la...

We needed an easy-to-use configuration language, and the only configuration language available at that time (1993) was Tcl. Our users did not consider Tcl an easy-to-use language. So we created our own configuration language.

Given how different Lua ended up, though, I imagine there was more to it than that.

Lua is much closer (not visually) to the C/Java family languages, which makes it more approachable.