I think the main reason for this is that these persons have simply spent more time thinking about cyronics compared to other people.

I don't think this is obvious at all. If you had asked me before in advance which of the following 4 possible sign-pairs would be true with increasing time spent thinking about cryonics:

1. less credence, less sign-ups
2. less credence, more sign-ups
3. more credence, more sign-ups
4. more credence, less sign-ups

I would have said 'obviously #3, since everyone starts from "that won't ever work" and move up from there, and then one is that much more likely to sign up'

The actual outcome, #2, would be the one I would expect least. (Hence, I am strongly suspicious of anyone claiming to expect or predict it as suffering from hindsight bias.)