My understanding is that there was a language fork after 5.1. One thing was a complete reworking of how math works. It used to be just floating point for everything but the new idea was to make it like Python 3. So most operations are float/integer with some weird exceptions.
As with any language fork there will be some who stay and others who switch to the new thing. Often a fork will drive people away from a particular language as in my case.
Lua's nature as a primarily embedded language means backwards compatibility is not guaranteed between any version. If 5.2 was a language fork then so was 5.3, 5.4, 5.5, etc. (5.2 did have some more significant changes though)
For that reason luajit staying at ~5.1 actually works in its favor. Rather than trying to follow the moving target of the newest version, it gives a robust focal point for the lua ecosystem, while modern versions can be developed and continue to serve their purpose in embedded systems and whatnot where appropriate.
I don't see a reason not to update LuaJIT still. Changes in Lua aren't just version numbers, it should be improving something, meaning that would be missing in LuaJIT.
LuaJIT does have some backported features from newer versions. But Mike Pall -- the mad genius behind LuaJIT -- has made it clear he doesn't agree with many of the changes made in newer versions, hence why it's staying where it's at.
In RHEL I would never touch system python at all, and would install what every version I needed in a venv and configure any software I installed to use what ever version I needed. I learned the hard way to never mess with system python.
As with any language fork there will be some who stay and others who switch to the new thing. Often a fork will drive people away from a particular language as in my case.