I always thought of it to be something like this; after their failed take over they were basically told "you want to live the life of the offlanders? well fine, go now and dont come back!"
Why they didnt come back is the big question, the Ba'ku didn't know they or Starfleet were there till Data went mad. The other thing is why did the Son'a need Federation or Starfleet help?
If the Briar patch mucked up communications and sensors like the film suggested couldnt they just sneak in, use their collector and then leave? Unless it is a long drawn out process which could lead to them being discovered later down the line?
For that matter why not simply come back, blast the colony from space and pop there every now and then long before the collector became an option? It must have been a recent development or they would have tried before.
I wonder how many of the Starfleet personnel observing knew what was going on? The Holoship was a Starfleet ship so some of them must have known what was going to happen even if Data didnt.