For many libraries, it's pretty easy to write code that works across all the versions. For many differences it's not too difficult to implement code for maximum compatibility: http://leafo.net/guides/setfenv-in-lua52-and-above.html

For me personally, the advantages of luajit currently outweigh the desire to upgrade to newer versions of the language.

Regarding Lua 5.1 being old, I haven't really found it to be an issue.. Lua isn't like most scripting languages, it doesn't come with a huge standard library. It's effectively just the language and some primitives. So it ages quite well as is.