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The target applications are SoC sub-blocks or small mixed-signal ASICs that require a compact and efficient microprocessor for programmable high-level control. There are many 32 or 64-bit RISC-V or ARM cores that try to fill this niche, but prove to be inefficient solutions given many of these applications only require 16-bit precision.

Unfortunately 6502, 8051, Z80, and various other architectures from a similar era already fill that niche.

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The 6809 is one of the architectures from the era of the 6502, 8051, and Z80. Specifically, it's Motorola's response to the 6502 eating the 6800's lunch, the same way the 8086 was Intel's response to the Z80 eating the 8080's lunch.

I'm guessing they picked the 6809 because it's a lot more friendly to C than those other architectures.

You're probably right. There's a lot of code for those processors that's not going away.

Too bad though, the 6809 is the pinnacle of 8-bit CPU architecture. It's the result of analyzing the practical shortcomings of the 6800 and other CPUs and redesigning the ISA. It has 16-bit index registers and 16-bit arithmetic, addressing two of the 6502s most annoying drawbacks. Additionally, it has support for stack based locals, position independent code and a LEA instruction. All those make building a high level language compiler much easier than other 8-bits.

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