Because the Son'a didn't want to live there.
Admiral Matthew Dougherty: With metaphasics, life spans will be doubled. An entire new medical science will evolve. I understand your chief engineer has the use of his eyes for the first time in his life. Would you take that away from him?
Jean-Luc Picard: There are metaphasic particles all over the Briar Patch. Why does it have to be this one planet?
Admiral Matthew Dougherty: It's the concentration in the rings that makes the whole damn thing work. Don't ask me to explain it. I only know they inject something into the rings that starts a thermolytic reaction. When it's over, the planet will be uninhabitable for generations.
Jean-Luc Picard: Admiral, delay the procedure. Let my people look at the technology.
Admiral Matthew Dougherty: Our best scientific minds already have. We can't find any other way to do this.
Jean-Luc Picard: Then the Son'a can establish a separate colony on the planet until we do.
Admiral Matthew Dougherty: It would take ten years of normal exposure to begin to reverse their condition. Some of them won't survive that long. Besides, they don't want to live in the middle of the Briar Patch. Who would?