Obi-Wan is visibly stunned by that, then seems to wilt a little "Oh... oh I see."

That seemed to imply an Imperial occupation of Mandalore. With Satine's assassination, and Maul creating unrest, there was undeniably a power struggle. One that the Empire must have taken advantage of and moved in.

"Satine would have never let that happen," he said quietly. But then, if she had still been alive, she might have been forced anyway. Perhaps it was a good thing she hadn't lived to see the rule of the Empire.