Once upon a time, I had a job where most of what I did involved signing up people for cryonics. I'm guessing that few other people on this site can say they've ever made a salary off that (unless you're reading this, Derek), and so I can speak with some small authority. Over those four excruciating years at Alcor, I spent hundreds of hours discussing the subject with hundreds of people.
Obviously I never came up with a definitive answer as to why some people get it and most don't. But I developed a working map of the conceptual space. Rather than a single "click," I found that there were a series of memetic filters.
The first and largest by far tended to be religious, which is to say, afterlife mythology. If you thought you were going to Heaven, Kolob, another plane of existence, or another body, you wouldn't bother investing the money or emotional effort in cryonics.
Only then came the intellectual barriers, but the boundary could be extremely vague. I think that the vast majority of people didnt have any trouble grasping the basic scientific arguments for cryonics; the actual logic filter always seemed relatively thin to me. Instead, people used their intellect to rationalize against cryonics, either motivated by existing beliefs (from one end) or by resulting anxieties (from the other).
Anxieties relating to cryonics tended to revolve around social situation and/or death. Some people identified so deeply with their current social situation, the idea of losing that situation (family, friends, standing, culture, etc.) was unthinkable. Others were afflicted by a sort of hypothetical survivor guilt; why did they deserve to live, when so many of their loved ones had died? Perhaps the majority were simply repulsed by any thought of death itself; most of them spent their lives trying not to think about the fact that we would die, and found it extremely depressing or disorienting when forced to confront that fact.
I don't think I could categorize the stages of approach to cryonics quite as neatly (and questionably) as the Kubler-Ross stages of dying. Clearly there was nothing inevitable about coming to accept cryonics, and approximately 90-95% of everyone I met never made it past the first filter. Even when people passed all of the memetic filters I've mentioned, they still had a tendency to become mired at the beginning or middle of their cryonics arrangements, floating in some sort of metastable emotional fog (starting cryonics arrangements felt like retreating from death, proceeding with them felt like approaching it).
Oh well, I haven't thought about this subject much since 1999. This is just my off-the-cuff memory of how I used to make a living.