That's fine. Freezing it at 5.1 (with some extensions) makes it stable. Not every language needs to move at the neck-breaking speed of, say, Javascript, pulling in every trend and bad idea that comes along for the sake of staying hip and exploding with unnecessary complexity and bloat. We need small, simple, sane languages.

If you want to use a more recent Lua version, of course that's available. If you want to use LuaJIT then you know exactly what you're dealing with and that there won't be any weird surprises in the future.