Because it's not a Ponzi scheme. If I want to run a Ponzi...

Because it's not a Ponzi scheme. If I want to run a Ponzi scheme, I would pick a much easy topic to start with, like superfund investing or online currency etc, these things are hard to investigate and there is no obvious products to see.
Sandalwood takes 15 years to grow, it needs huge land, equipment, water right, manufacturer, a lot labors, a lot investing up front. Investor can go to check on these trees at any time. They can check other buyers return. They can check sandalwood auction price.
No one would setup a Ponzi scheme by growing trees. It's too much investing, takes too long, too easy to be busted and not enough return.