I like the mockery explanation. Cryonics is as about as socially acceptable as furry fandom; if furries scraped up a few millions for some TV spots, do you think they would get more or less members in the long run? There is such a thing as bad publicity.

And existing cryonics members might be exasperated - money used for advertising is money not used for research or long-term sustainability (I hear Alcor runs at a loss).