I can give you a couple of theories, one is, because there are only anonymous internet testimonies, made by anonymous people, that may not actually exist. This is what is used to promote the product, and people are wising up to internet scams. (well some people at least).
JH would like you to believe that its because there is a huge Conspiracy against him, by "big pharma", the "devil", and various and sundry "evil entities".
I personally think its because the product does not do what JH and his believers, say it does, and they are simply to brain washed to admit it, or to embarrassed to admit they were scammed.
Another theory is that they've died from whatever disease they were trying to cure, (because Miracle-Mineral-Supplement is not a miracle cure), and are not around to tell people about it.