They answer this on the github:

> Unstable interfaces. An LLVM JIT is already used by Rubicon. A lot of efforts in preparation of code used by RTL insns (an environment)

https://github.com/vnmakarov/ruby/tree/rtl_mjit_branch#a-few...

llvm isn't always available, doesn't support as many architectures and doesn't always give the best performance.

The *nix philosophy has long been towards trying to provide choice wherever possible, so that people can use the tool that best meets their needs.

The new Ruby method JIT can use either GCC or clang as the backend. It uses C as an intermediate representation.