The 6809 is an amazing little processor, you can run multi-tasking and relocatable code on it with relative ease. And with some bank switching magic you can even do that with appreciable amounts of RAM for each task. It is also one of the few instruction sets that is very predictable, if you know some base formats then you can 'compose' instructions and they usually exist as a valid opcode.

“UniFLEX is a Unix-like operating system ... for the Motorola 6809”:

https://en.m.wikipedia.org/wiki/UniFLEX

I haven't used the 6800 family, but I'd expect most people would have preferred them - the motivation for the 6502 was to drastically cut cost, and while a lot was achieved by an amazing design, it was also a number of feature trade-offs. The main designers of the 6502 (Mensch and Peddle) were both on the team that worked on the 6800, and Peddle pitched the 65xx proposal to Motorola first, before taking it to MOS when Motorola was more concerned about protecting their margins. Before the 6501 and 6502 hit the market, a 6800 cost $175. 6501 and 6502 were introduced for $20 and $25 respectively. A year later the 6800 cost $35.

The price adjustments are so fast (and, checking the Wikipedia page, it looks like they managed to squeeze in a lawsuit and a settlement in the same time period!), it makes you wonder what it was beside price that made the 6502 so popular.

Commodore bought MOS, and managed to squeeze margins out of MOS that were essential to their ability to get the PET out as cheaply as they did.

Combine that with the Apple I and II, and the 6502 was a major player, and the 6800 didn't have any massively compelling benefits.

The 6809 did have benefits over the 6502, and did get some design wins, including Commodore's SuperPET (which bizarrely had both a 6809 and a 6502), but I think the 6809 was too late - on the low end cheap machines like the VIC-20 and then C64 completely trounced the 6809 based machines, and so it just didn't get enough mindshare, and just a few years later it was effectively too late for it to get any traction in the home computer market.

I like the 6800. Two accumulators and a 16 bit index register is a good alternative. Totally understand your preference. The 6805 seems too cut down.

I have not written much 6800 code, but have read a fair bit. Had it been more available to me, I would definitely enjoyed it

The 6809 is the Cadillac of 8 bitters. That is what makes it fun. One can pack a ton of features into small spaces and doing reentrant, relocatable code is very well supported.

Stack abuse gets one a really fast memory to memory move too.