For extra fun, "flang" actually refers to three different projects, one of which also completely rebuilt its frontend, so there are four separate "flang" frontends to LLVM. And I know of at least one proprietary LLVM-based Fortran compiler that's not based on any of the flags, and I suspect there are a few more lurking about.

The short answer as to why there are so many different LLVM Fortran frontends is that open source community took a long time to find someone willing to commit to a Fortran frontend to LLVM, and even when that happened, the initial results weren't particularly usable--it's not until I want to say late 2020 or early 2021 that there's an in-LLVM-tree flang project, and the exe isn't renamed from flang-new to flang until 2024.