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.
