It is mentioned in the film that the Son'a didn't want to set up a colony inside the Briar Patch where they would be isolated. They always wanted to take over, extract the radiation and play the game with interstellar powers.
It's all in the film.
So is the the fact that they were expelled by the Ba'ku. Seems a nifty trick for a group of Amish.
There's more than 100 years between that and the events of Insurrection. Maybe they did still use their technology to greater levels than that. The Son'a didn't develop those psychic abilities the Ba'ku had developed, for instance. So there's reason to assume that the Ba'ku evolved even further in their "mind over muscle" attitude.
AND the Ba'ku didn't realize who the Son'a really were until it was to late. Picard only told them when they were all inside the Son'a ship. You and I don't know how they would have reacted had they recognized who the Son'a were to begin with. Then it wouldn't have been a fight between them and some offworlders. Maybe then they would have dusted off some of their space ships and weapons. No way to know.
What we DO know is that they could take a look at Data's positronic brain in an instant, and the statement that they can have warp drive any time when they feel like it. And that the Son'a didn't reveal their true nature. And that they had to rely on Federation cloaking technology to return to the planet. Sounds to me like they were pretty afraid of the Ba'ku.