Many writers of the secular/humanist variety are dedicated to such
conventional ideals as unselfishness and sacrifice. In some cases,
that may be why they are able to devote so much energy to the efforts
which have brought them to your attention. Of course, they are "only
human", and so they have more personal goals as well, but they often
view these as flaws, and aspire to identify more with the group's goals
than with their own individual ones. To them, cryonics is a crime of
the highest order. By striving to avoid death, the great leveler,
practitioners are in effect setting themselves above the rest of human
society.
The result is that the typical secular humanist is one of the least likely
candidates for cryonics. This would apply to Asimov and Sagan in the
list above. I seem to recall some claims that Feynman was skeptical about
the workability of the idea. (If true, as one of the founders of the idea
of nanotechnology this should give us all pause.) Clarke does not come
across as a bleeding-heart liberal like Asimov, so I don't know what his
thoughts would be about it.
Hal