My understanding is that the number of people signed up is in the thousands, which if it is correct means probably a bit less than one in a million persons.

You might have meant it rhetorically, but if it is true that it is a "big mystery" to you why most people have not signed up, then your best guess for the reason for this should be that signing up for cryonics is foolish and useless, just as if a patient in a psychological ward finds himself thinking, "I wonder why so few people say they are Napoleon?", his best guess should be that the reason for this is that the people he knows, including himself, are not in fact Napoleon.

As another example, if you are at the airport and you see two lines while you are checking in, a very long one and a very short one, and you say, "It's a big mystery to me why so many people are going in that long line instead of the short one," then you'd better get in that long line, because if you get in the short one, you are going to find yourself kicked out of it. On the other hand if you do know the reasons, you may be able to get in the short line.

In the cryonics case, this is pretty much true no matter how convincing you find your reasons, until you can understand why people do not sign up.