It could easily keep up with Lua if Mike Pall agreed with the changes made to Lua, but he didn't.

The Lua ecosystem is very different than almost any other; It's primary use case, probably 95% of projects using it or more, is being embedded into another project for scripting/configuration/control. As such, it is common for projects to pick a version of Lua and stick with it, rather than upgrade to the latest-and-greatest-with-slight-to-major-incompatibilities. Pall liked 5.2, and thought 5.3 didn't offer enough to break compatibility.