[SubjectToChange]

The problem is that "weird CPUs and embedded systems" are not mainstream platforms, nor do they have a lot of money behind them (particularly for open source development). Hence, there is little motivation and/or resources for anyone to develop a new backend for LLVM when a mature GCC backend already exists. Moreover, the LLVM developers are weary to accept new backends for niche platforms when there is no guarantee that they will be maintained in the future.

[bombcar]

Exactly - often the GCC branch is barely maintained by whoever is selling the CPU/chip, and it is patches against a specific version that you must use, or it goes haywire.

At least with GCC they can sometimes merge the patches.