I am a donk at cash games, even more so because I play turbos which get shallower stacked even faster than normals, leaving even less scope for postflop play. I suck at cash games when I play them (although I am trying to improve).

I think the points you made are the main ones, particularly that about shallower stacks.

That said, I don't think it's fair to say that "cash players are generally better". It's a different set of skills to play cash vs tourneys. There are plenty of cash game players who suck at tourneys when they come to play them.