As applies specifically to cryonics, I remember my sole rejection was a lack of affordability, and it turned out I was wrong about that. I think the first time I heard about the possibility I was 9 years old, and it clicked for me then -- but until just a few weeks ago, I was laboring under the misconception that you basically have to be rich to get it. That click can be painful when it generates conflicts between your utility function and the circumstances of your life.

I wonder if that might not lead some people to reject cryonics who'd otherwise be amenable, on the basis of false information -- to want it is painful or feels like a desire unfulfillable, so they seek to eliminate the dissonance by finding reasons it's not truly possible/desireable. Certainly this doesn't seem to explain the typical rejection I've encountered very well, but I can think of a number of geeky/intellectual types (until recently, m'self included) whose main issue seemed to be that it didn't seem within reach.

(And on that note, nervous about hearing back on life insurance quotes -- I'm poor, so if for some reason my health issues render me uninsurable, it's gonna be pretty painful. Facing that uncertainty very nearly stopped me from applying anytime soon even after I'd found out it might in principle be possible!)