I don't have much experience with writing a custom gcc backend, but my experience with LLVM is that its model ends up being a somewhat poor fit for smaller or weirder architectures. For example, LLVM has this annoying tendency to really aggressively promote the size of everything--if you have a 32-bit core with 64-bit memory indexing, LLVM tends to make anything that eventually becomes a memory index a 64-bit computation, even if it would be more efficient to keep everything as 32-bit.

Hm, is it so much easier to write an LLVM back-end than a GCC back-end? I haven't looked into GCC's code gen infrastructure that much, but I looked into making a new back-end to LLVM some time ago and it seemed extremely complex and entirely undocumented. All guidance I found on it basically amounted to, "copy an existing back-end and bang it against a rock until you've made it generate the machine code you need". And it certainly wasn't a case of having clean and simple enough code to make documentation unnecessary. It would honestly surprise me if GCC is more difficult to add a back-end to.

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.