I'd say that may have something to do with it. But I was thinking more along the lines of game engines utilizing DX11 to their fullest typically show up down the road and then games use that take time to be developed. DX10 has been out for how many years? Is it a major driving force in game development yet??? It took years for DX9 to become mainstream. It is the nature of the beast. And on a low end part? Outside of the bullet point is really shouldnt be a major concern when making a purchase today.
But there's a lot of reasons DX10 was never used:
1. Console ports
2. Noone bought into Vista
3. Nvidia forced MS to nerf the hell out of DX10 to the point it was basically DX9 with different memory management
4. You had to write a DX9 engine and a dx10 engine to cover both bases
Reason why DX11 should be more prevalent:
1. Next gen of consoles isn't too far out
2. Vista and Win7 are both well established now
3. DX11 has everything DX10 was suppose to have and everything that was wanted for DX11 too.
4. DX11 uses a compatibility switch so one DX11 engine can operate on DX9, DX10 or DX11 hardware, meaning no reason not to write in DX11 and let it switch down to DX9 mode.