First, because I knew Io and not Lua. I had gotten interested in Io years before. Io was actually my first dynamic language, not Python or Ruby or Javascript. I stopped using Io for a few years when I moved on to other things, but _why's article "Io Has a Very Clean Mirror" brought it back to mind.

I seriously considered Lua but I felt object orientation needs to be incorporated very fundamentally in a language. I had previous experience with mIRC which shares Lua's idea of "everything is a table", and while mIRC is no doubt productive I felt tables would be a step backwards compared to Io's objects.

At the time I started, the C/C++ binding frameworks for Lua were not as mature as they are now, not as numerous, or I simply wasn't aware of them.

From a language aesthetic standpoint my decision came down to Io, or Ruby, but I looked at the C Ruby source code and it didn't seem very suited to embedding. For instance the official implementation of Ruby had some dependence on global/static variables which precluded the idea of running multiple instances of the VM in a process. Io allows this, although I haven't used it yet; I plan to use it to support threading, one VM per thread. And basically the Ruby interpreter codebase wants to own main() and call my code as a library; I wanted to own main() and call the interpreter as a library. Io supports this, and it's codebase is smaller.