It's a design choice. Lua was first intended to be a configuration language for engineers to enter data at an oil company. They were used to the 1st thing being number 1, the 2nd thing being number 2, and so forth. It's just a very natural way of counting.

You don't change something like that because it eventually got picked up by game programmers (never the intent of Lua, something that just happened after it was used by the Grim Fandango team, then it took off in the gaming world).