why was jackson's island chosen by the pirates or the boys?

Jackson Island was chosen because it was uninhabited.

Three miles below St. Petersburg, at a point where the Mississippi River was a trifle over a mile wide, there was a long, narrow, wooded island, with a shallow bar at the head of it, and this offered well as a rendezvous. It was not inhabited; it lay far over toward the further shore, abreast a dense and almost wholly unpeopled forest. So Jackson's Island was chosen.