An important part of this is that Lua is generally embedded into larger programs. This is pretty much the #1 production-grade use case of the language. So when a new version of Lua is released, no one is actually expected to update their old versions. Rather, they use the new version in future projects. This allows the language designers to improve the language in ways that are backwards incompatible (imagine if JS could do this; the `let` keyword wouldn't even exist because you could just fix the default scoping).

So Lua 5.3 has a lot of breaking changes without a clear upgrade path. LuaJIT simply isn't going to support this, because its users generally use it more like a normal scripting language like Python and expect more stability.