Ahsoka was presumed dead two or three years before A New Hope and probably came back some time during the OT. We also get a good look at what type of missions the Ghost crew (and therefore Sabine) take part in in Rebels, and those are usually more low-key like supply runs. Even with Hera climbing the ranks of the Rebellion, it makes sense we wouldn’t hear much of Sabine and Ahsoka, because they aren’t working directly with the main characters.